feat: Add metrics

Fixes #7.

Signed-off-by: Julien Riou <julien@riou.xyz>

src/cmd/collerd/README.md

@@ -27,5 +27,9 @@ The file format is **JSON**:
 * **expiration** (int): Default expiration time in seconds
 * **max_upload_size** (int): Maximum number of bytes received by the server for notes (the base64 encoding may use more space than the real size of the note)
 * **show_version** (bool): Show version on the website
+* **enable_metrics** (bool): Enable Prometheus endpoint (default false)
+* **prometheus_route** (string): Route to expose Prometheus metrics (default "/metrics")
+* **prometheus_notes_metric** (string): Name of the notes count metric (default "collerd_notes")
+* **observation_internal** (int): Number of seconds to wait between two observations (default 60)
 
 The configuration file is not required but the service might not be exposed to the public.

src/cmd/collerd/main.go

@@ -7,6 +7,7 @@ import (
 
 	"git.riou.xyz/jriou/coller/internal"
 	"git.riou.xyz/jriou/coller/server"
+	"github.com/prometheus/client_golang/prometheus"
 )
 
 var (
@@ -75,6 +76,16 @@ func handleMain() int {
 	srv.SetIDLength(config.IDLength)
 	srv.SetPasswordLength(config.PasswordLength)
 
+	if config.EnableMetrics {
+		reg := prometheus.NewRegistry()
+		metrics, err := server.NewMetrics(logger, reg, config, db)
+		if err != nil {
+			logger.Error("could not register metrics", slog.Any("error", err))
+			return internal.RC_ERROR
+		}
+		srv.SetMetrics(metrics)
+	}
+
 	err = srv.Start()
 	if err != nil {
 		logger.Error("could not start server", slog.Any("error", err))

src/go.mod

@@ -6,6 +6,7 @@ toolchain go1.24.6
 
 require (
 	github.com/gorilla/mux v1.8.1
+	github.com/prometheus/client_golang v1.23.0
 	golang.design/x/clipboard v0.7.1
 	golang.org/x/crypto v0.41.0
 	golang.org/x/term v0.34.0
@@ -15,6 +16,8 @@ require (
 )
 
 require (
+	github.com/beorn7/perks v1.0.1 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/jackc/pgpassfile v1.0.0 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
 	github.com/jackc/pgx/v5 v5.6.0 // indirect
@@ -22,10 +25,15 @@ require (
 	github.com/jinzhu/inflection v1.0.0 // indirect
 	github.com/jinzhu/now v1.1.5 // indirect
 	github.com/mattn/go-sqlite3 v1.14.22 // indirect
+	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
+	github.com/prometheus/client_model v0.6.2 // indirect
+	github.com/prometheus/common v0.65.0 // indirect
+	github.com/prometheus/procfs v0.16.1 // indirect
 	golang.org/x/exp/shiny v0.0.0-20250606033433-dcc06ee1d476 // indirect
 	golang.org/x/image v0.28.0 // indirect
 	golang.org/x/mobile v0.0.0-20250606033058-a2a15c67f36f // indirect
 	golang.org/x/sync v0.16.0 // indirect
 	golang.org/x/sys v0.35.0 // indirect
 	golang.org/x/text v0.28.0 // indirect
+	google.golang.org/protobuf v1.36.6 // indirect
 )

src/go.sum

@@ -1,6 +1,12 @@
+github.com/beorn7/perks v1.0.1 h1:VlbKKnNfV8bJzeqoa4cOKqO6bYr3WgKZxO8Z16+hsOM=
+github.com/beorn7/perks v1.0.1/go.mod h1:G2ZrVWU2WbWT9wwq4/hrbKbnv/1ERSJQ0ibhJ6rlkpw=
+github.com/cespare/xxhash/v2 v2.3.0 h1:UL815xU9SqsFlibzuggzjXhog7bL6oX9BbNZnL2UFvs=
+github.com/cespare/xxhash/v2 v2.3.0/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
+github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
 github.com/gorilla/mux v1.8.1 h1:TuBL49tXwgrFYWhqrNgrUNEY92u81SPhu7sTdzQEiWY=
 github.com/gorilla/mux v1.8.1/go.mod h1:AKf9I4AEqPTmMytcMc0KkNouC66V3BtZ4qD5fmWSiMQ=
 github.com/jackc/pgpassfile v1.0.0 h1:/6Hmqy13Ss2zCq62VdNG8tM1wchn8zjSGOBJ6icpsIM=
@@ -15,15 +21,31 @@ github.com/jinzhu/inflection v1.0.0 h1:K317FqzuhWc8YvSVlFMCCUb36O/S9MCKRDI7QkRKD
 github.com/jinzhu/inflection v1.0.0/go.mod h1:h+uFLlag+Qp1Va5pdKtLDYj+kHp5pxUVkryuEj+Srlc=
 github.com/jinzhu/now v1.1.5 h1:/o9tlHleP7gOFmsnYNz3RGnqzefHA47wQpKrrdTIwXQ=
 github.com/jinzhu/now v1.1.5/go.mod h1:d3SSVoowX0Lcu0IBviAWJpolVfI5UJVZZ7cO71lE/z8=
+github.com/klauspost/compress v1.18.0 h1:c/Cqfb0r+Yi+JtIEq73FWXVkRonBlf0CRNYc8Zttxdo=
+github.com/klauspost/compress v1.18.0/go.mod h1:2Pp+KzxcywXVXMr50+X0Q/Lsb43OQHYWRCY2AiWywWQ=
+github.com/kylelemons/godebug v1.1.0 h1:RPNrshWIDI6G2gRW9EHilWtl7Z6Sb1BR0xunSBf0SNc=
+github.com/kylelemons/godebug v1.1.0/go.mod h1:9/0rRGxNHcop5bhtWyNeEfOS8JIWk580+fNqagV/RAw=
 github.com/mattn/go-sqlite3 v1.14.22 h1:2gZY6PC6kBnID23Tichd1K+Z0oS6nE/XwU+Vz/5o4kU=
 github.com/mattn/go-sqlite3 v1.14.22/go.mod h1:Uh1q+B4BYcTPb+yiD3kU8Ct7aC0hY9fxUwlHK0RXw+Y=
+github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 h1:C3w9PqII01/Oq1c1nUAm88MOHcQC9l5mIlSMApZMrHA=
+github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822/go.mod h1:+n7T8mK8HuQTcFwEeznm/DIxMOiR9yIdICNftLE1DvQ=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/prometheus/client_golang v1.23.0 h1:ust4zpdl9r4trLY/gSjlm07PuiBq2ynaXXlptpfy8Uc=
+github.com/prometheus/client_golang v1.23.0/go.mod h1:i/o0R9ByOnHX0McrTMTyhYvKE4haaf2mW08I+jGAjEE=
+github.com/prometheus/client_model v0.6.2 h1:oBsgwpGs7iVziMvrGhE53c/GrLUsZdHnqNwqPLxwZyk=
+github.com/prometheus/client_model v0.6.2/go.mod h1:y3m2F6Gdpfy6Ut/GBsUqTWZqCUvMVzSfMLjcu6wAwpE=
+github.com/prometheus/common v0.65.0 h1:QDwzd+G1twt//Kwj/Ww6E9FQq1iVMmODnILtW1t2VzE=
+github.com/prometheus/common v0.65.0/go.mod h1:0gZns+BLRQ3V6NdaerOhMbwwRbNh9hkGINtQAsP5GS8=
+github.com/prometheus/procfs v0.16.1 h1:hZ15bTNuirocR6u0JZ6BAHHmwS1p8B4P6MRqxtzMyRg=
+github.com/prometheus/procfs v0.16.1/go.mod h1:teAbpZRB1iIAJYREa1LsoWUXykVXA1KlTmWl8x/U+Is=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
 github.com/stretchr/testify v1.3.0/go.mod h1:M5WIy9Dh21IEIfnGCwXGc5bZfKNJtfHm1UVUgZn+9EI=
 github.com/stretchr/testify v1.7.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
-github.com/stretchr/testify v1.8.1 h1:w7B6lhMri9wdJUVmEZPGGhZzrYTPvgJArz7wNPgYKsk=
+github.com/stretchr/testify v1.10.0 h1:Xv5erBjTwe/5IxqUQTdXv5kgmIvbHo3QQyRwhJsOfJA=
-github.com/stretchr/testify v1.8.1/go.mod h1:w2LPCIKwWwSfY2zedu0+kehJoqGctiVI29o6fzry7u4=
+github.com/stretchr/testify v1.10.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
+go.uber.org/goleak v1.3.0 h1:2K3zAYmnTNqV73imy9J1T3WC+gmCePx2hEGkimedGto=
+go.uber.org/goleak v1.3.0/go.mod h1:CoHD4mav9JJNrW/WLlf7HGZPjdw8EucARQHekz1X6bE=
 golang.design/x/clipboard v0.7.1 h1:OEG3CmcYRBNnRwpDp7+uWLiZi3hrMRJpE9JkkkYtz2c=
 golang.design/x/clipboard v0.7.1/go.mod h1:i5SiIqj0wLFw9P/1D7vfILFK0KHMk7ydE72HRrUIgkg=
 golang.org/x/crypto v0.41.0 h1:WKYxWedPGCTVVl5+WHSSrOBT0O8lx32+zxmHxijgXp4=
@@ -42,6 +64,8 @@ golang.org/x/term v0.34.0 h1:O/2T7POpk0ZZ7MAzMeWFSg6S5IpWd/RXDlM9hgM3DR4=
 golang.org/x/term v0.34.0/go.mod h1:5jC53AEywhIVebHgPVeg0mj8OD3VO9OzclacVrqpaAw=
 golang.org/x/text v0.28.0 h1:rhazDwis8INMIwQ4tpjLDzUhx6RlXqZNPEM0huQojng=
 golang.org/x/text v0.28.0/go.mod h1:U8nCwOR8jO/marOQ0QbDiOngZVEBB7MAiitBuMjXiNU=
+google.golang.org/protobuf v1.36.6 h1:z1NpPI8ku2WgiWnf+t9wTPsn6eP1L7ksHUlkfLvd9xY=
+google.golang.org/protobuf v1.36.6/go.mod h1:jduwjTPXsFjZGTmRluh+L6NjiWu7pchiJ2/5YcXBHnY=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=

src/server/config.go

@@ -19,6 +19,10 @@ type Config struct {
 	Expiration            int    `json:"expiration"`
 	MaxUploadSize         int64  `json:"max_upload_size"`
 	ShowVersion           bool   `json:"show_version"`
+	EnableMetrics         bool   `json:"enable_metrics"`
+	PrometheusRoute       string `json:"prometheus_route"`
+	PrometheusNotesMetric string `json:"prometheus_notes_metric"`
+	ObservationInterval   int    `json:"observation_internal"`
 }
 
 func NewConfig() *Config {
@@ -41,6 +45,10 @@ func NewConfig() *Config {
 		Expiration:            604800,   // 7 days
 		MaxUploadSize:         10485760, // 10MiB (encoded)
 		ShowVersion:           false,
+		EnableMetrics:         false,
+		PrometheusRoute:       "/metrics",
+		PrometheusNotesMetric: "collerd_notes",
+		ObservationInterval:   60,
 	}
 }
 

src/server/metrics.go

@@ -0,0 +1,55 @@
+package server
+
+import (
+	"fmt"
+	"log/slog"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+type Metrics struct {
+	logger   *slog.Logger
+	notes    prometheus.Gauge
+	db       *Database
+	reg      *prometheus.Registry
+	interval int
+}
+
+func NewMetrics(logger *slog.Logger, reg *prometheus.Registry, config *Config, db *Database) (*Metrics, error) {
+	l := logger.With("module", "metrics")
+	m := &Metrics{
+		logger: l,
+		db:     db,
+		notes: prometheus.NewGauge(prometheus.GaugeOpts{
+			Name: config.PrometheusNotesMetric,
+		}),
+		reg:      reg,
+		interval: config.ObservationInterval,
+	}
+	if err := reg.Register(m.notes); err != nil {
+		return nil, err
+	}
+
+	go m.ObserveThread()
+
+	return m, nil
+}
+
+func (m *Metrics) ObserveThread() {
+	m.logger.Debug("starting observations")
+	for {
+		m.logger.Debug("counting notes")
+		var count int64
+		m.db.db.Model(&Note{}).Count(&count)
+		m.notes.Set(float64(count))
+		m.logger.Debug("counted notes", slog.Any("count", count))
+
+		wording := "second"
+		if m.interval > 1 {
+			wording += "s"
+		}
+		m.logger.Debug(fmt.Sprintf("waiting for %d %s before next observation", m.interval, wording))
+		time.Sleep(time.Duration(m.interval) * time.Second)
+	}
+}

src/server/server.go

@@ -14,6 +14,7 @@ import (
 
 	"git.riou.xyz/jriou/coller/internal"
 	"github.com/gorilla/mux"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
 var passwordLength = internal.MIN_PASSWORD_LENGTH
@@ -23,6 +24,7 @@ type Server struct {
 	db      *Database
 	config  *Config
 	version string
+	metrics *Metrics
 }
 
 func NewServer(logger *slog.Logger, db *Database, config *Config, version string) (*Server, error) {
@@ -44,6 +46,10 @@ func (s *Server) SetPasswordLength(length int) {
 	passwordLength = length
 }
 
+func (s *Server) SetMetrics(metrics *Metrics) {
+	s.metrics = metrics
+}
+
 type ErrorResponse struct {
 	Message string `json:"message"`
 	Error   string `json:"error"`
@@ -312,8 +318,15 @@ var templatesFS embed.FS
 func (s *Server) Start() error {
 	r := mux.NewRouter().StrictSlash(true)
 
-	// API
+	// Healthchecks
 	r.HandleFunc("/health", HealthHandler)
+
+	// Metrics
+	if s.metrics != nil && s.metrics.reg != nil {
+		r.Path(s.config.PrometheusRoute).Handler(promhttp.HandlerFor(s.metrics.reg, promhttp.HandlerOpts{Registry: s.metrics.reg})).Methods("GET")
+	}
+
+	// API
 	r.Path("/api/note").Handler(&CreateNoteHandler{logger: s.logger, db: s.db, maxUploadSize: s.config.MaxUploadSize}).Methods("POST")
 	r.Path("/{id:[a-zA-Z0-9]+}/{password:[a-zA-Z0-9]+}").Handler(&GetProtectedNoteHandler{logger: s.logger, db: s.db}).Methods("GET")
 	r.Path("/{id:[a-zA-Z0-9]+}").Handler(&GetNoteHandler{logger: s.logger, db: s.db}).Methods("GET")