#20 add prometheus metrics endpoint

cmd/cache/main.go

@@ -3,11 +3,13 @@ package main
 import (
 	"context"
 	"errors"
+	"fmt"
 	"log"
 	"net/http"
 	"time"
 
 	"github.com/kelseyhightower/envconfig"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"go.uber.org/zap"
 	"go.uber.org/zap/zapcore"
 	goahttp "goa.design/goa/v3/http"
@@ -106,6 +108,9 @@ func main() {
 	goahealthsrv.Mount(mux, healthServer)
 	goaopenapisrv.Mount(mux, openapiServer)
 
+	// expose metrics
+	go exposeMetrics(cfg.Metrics.Addr, logger)
+
 	var handler http.Handler = mux
 	srv := &http.Server{
 		Addr:         cfg.HTTP.Host + ":" + cfg.HTTP.Port,
@@ -150,3 +155,12 @@ func createLogger(logLevel string, opts ...zap.Option) (*zap.Logger, error) {
 func errFormatter(e error) goahttp.Statuser {
 	return service.NewErrorResponse(e)
 }
+
+func exposeMetrics(addr string, logger *zap.Logger) {
+	promMux := http.NewServeMux()
+	promMux.Handle("/metrics", promhttp.Handler())
+	logger.Info(fmt.Sprintf("exposing prometheus metrics at %s/metrics", addr))
+	if err := http.ListenAndServe(addr, promMux); err != nil {
+		logger.Error("error exposing prometheus metrics", zap.Error(err))
+	}
+}

go.mod

@@ -8,6 +8,7 @@ require (
 	github.com/cloudevents/sdk-go/v2 v2.10.1
 	github.com/go-redis/redis/v8 v8.11.5
 	github.com/kelseyhightower/envconfig v1.4.0
+	github.com/prometheus/client_golang v1.12.2
 	github.com/stretchr/testify v1.7.0
 	go.uber.org/zap v1.21.0
 	goa.design/goa/v3 v3.7.0
@@ -15,22 +16,28 @@ require (
 )
 
 require (
+	github.com/beorn7/perks v1.0.1 // indirect
 	github.com/cespare/xxhash/v2 v2.1.2 // indirect
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/dgryski/go-rendezvous v0.0.0-20200823014737-9f7001d12a5f // indirect
 	github.com/dimfeld/httppath v0.0.0-20170720192232-ee938bf73598 // indirect
 	github.com/dimfeld/httptreemux/v5 v5.4.0 // indirect
+	github.com/golang/protobuf v1.5.2 // indirect
 	github.com/google/go-cmp v0.5.6 // indirect
 	github.com/gorilla/websocket v1.5.0 // indirect
-	github.com/json-iterator/go v1.1.11 // indirect
+	github.com/json-iterator/go v1.1.12 // indirect
 	github.com/jtolds/gls v4.20.0+incompatible // indirect
 	github.com/manveru/faker v0.0.0-20171103152722-9fbc68a78c4d // indirect
-	github.com/modern-go/concurrent v0.0.0-20180228061459-e0a39a4cb421 // indirect
-	github.com/modern-go/reflect2 v1.0.1 // indirect
+	github.com/matttproud/golang_protobuf_extensions v1.0.1 // indirect
+	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
+	github.com/modern-go/reflect2 v1.0.2 // indirect
 	github.com/nats-io/nats.go v1.11.1-0.20210623165838-4b75fc59ae30 // indirect
 	github.com/nats-io/nkeys v0.3.0 // indirect
 	github.com/nats-io/nuid v1.0.1 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/prometheus/client_model v0.2.0 // indirect
+	github.com/prometheus/common v0.32.1 // indirect
+	github.com/prometheus/procfs v0.7.3 // indirect
 	github.com/sergi/go-diff v1.2.0 // indirect
 	github.com/zach-klippenstein/goregen v0.0.0-20160303162051-795b5e3961ea // indirect
 	go.uber.org/atomic v1.7.0 // indirect
@@ -40,6 +47,7 @@ require (
 	golang.org/x/sys v0.0.0-20220317061510-51cd9980dadf // indirect
 	golang.org/x/tools v0.1.10 // indirect
 	golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1 // indirect
+	google.golang.org/protobuf v1.27.1 // indirect
 	gopkg.in/yaml.v2 v2.4.0 // indirect
 	gopkg.in/yaml.v3 v3.0.0-20210107192922-496545a6307b // indirect
 )

internal/config/config.go

@@ -3,9 +3,10 @@ package config
 import "time"
 
 type Config struct {
-	HTTP  httpConfig
-	Redis redisConfig
-	Nats  natsConfig
+	HTTP    httpConfig
+	Redis   redisConfig
+	Nats    natsConfig
+	Metrics metricsConfig
 
 	LogLevel string `envconfig:"LOG_LEVEL" default:"INFO"`
 }
@@ -30,3 +31,7 @@ type natsConfig struct {
 	Addr    string `envconfig:"NATS_ADDR" required:"true"`
 	Subject string `envconfig:"NATS_SUBJECT" default:"external"`
 }
+
+type metricsConfig struct {
+	Addr string `envconfig:"METRICS_ADDR" default:":2112"`
+}