measure GitHub API rate limit

This will provide visibility on how much GitHub API v3 and v4
rate limit is remaining at various points in time, and how it
changes when services are added or modified.

main.go

@@ -228,6 +228,8 @@ func run(ctx context.Context, cancel context.CancelFunc, storeDir, stateFile, an
 
 	if *metricsHTTPFlag != "" {
 		initMetrics(cancel, *metricsHTTPFlag)
+		go measureGitHubV3RateLimit()
+		go measureGitHubV4RateLimit()
 	}
 
 	staticFiles := cookieAuth{httpgzip.FileServer(

metrics.go

@@ -11,22 +11,32 @@ import (
 
 type prometheusMetrics struct {
 	goGetRequestsTotal *prometheus.CounterVec
+	githubRateLimit    *prometheus.GaugeVec
 }
 
 var metrics = &prometheusMetrics{
 	goGetRequestsTotal: prometheus.NewCounterVec(prometheus.CounterOpts{
 		Name: "home_go_get_requests_total",
 		Help: "Total number of ?go-get=1 requests.",
 	}, []string{"path"}),
+	githubRateLimit: prometheus.NewGaugeVec(prometheus.GaugeOpts{
+		Name: "home_github_rate_limit",
+		Help: "Remaining requests the GitHub client can make this hour.",
+	}, []string{"client"}),
 }
 
 func (m *prometheusMetrics) IncGoGetRequestsTotal(importPath string) {
 	m.goGetRequestsTotal.With(prometheus.Labels{"path": importPath}).Inc()
 }
 
+func (m *prometheusMetrics) SetGitHubRateLimit(clientName string, remaining int) {
+	m.githubRateLimit.With(prometheus.Labels{"client": clientName}).Set(float64(remaining))
+}
+
 func initMetrics(cancel context.CancelFunc, httpAddr string) {
 	r := prometheus.NewRegistry()
 	r.MustRegister(metrics.goGetRequestsTotal)
+	r.MustRegister(metrics.githubRateLimit)
 
 	mux := http.NewServeMux()
 	mux.Handle("/metrics", promhttp.HandlerFor(r, promhttp.HandlerOpts{}))

users.go

@@ -36,6 +36,35 @@ var dmitshurPublicRepoGHV3, dmitshurPublicRepoGHV4 = func() (*githubv3.Client, *
 		githubv4.NewClient(&http.Client{Transport: authTransport, Timeout: 10 * time.Second})
 }()
 
+// measureGitHubV3RateLimit measures and reports GitHub API v3 rate limit.
+func measureGitHubV3RateLimit() {
+	for {
+		rate, _, err := dmitshurPublicRepoGHV3.RateLimits(context.Background())
+		if err != nil {
+			log.Println("dmitshurPublicRepoGHV3.RateLimits:", err)
+			time.Sleep(time.Minute)
+			continue
+		}
+		metrics.SetGitHubRateLimit("dmitshur-v3", rate.Core.Remaining)
+		time.Sleep(time.Minute)
+	}
+}
+
+// measureGitHubV4RateLimit measures and reports GitHub API v4 rate limit.
+func measureGitHubV4RateLimit() {
+	for {
+		var q struct{ RateLimit struct{ Remaining int } }
+		err := dmitshurPublicRepoGHV4.Query(context.Background(), &q, nil)
+		if err != nil {
+			log.Println("dmitshurPublicRepoGHV4.Query for RateLimit:", err)
+			time.Sleep(time.Minute)
+			continue
+		}
+		metrics.SetGitHubRateLimit("dmitshur-v4", q.RateLimit.Remaining)
+		time.Sleep(time.Minute)
+	}
+}
+
 type userCreator interface {
 	// Create creates the specified user.
 	// It returns os.ErrExist if the user already exists.