Add metrics for GitHub API rate limit (#312)

github/metrics/transport.go

@@ -0,0 +1,63 @@
+// Package metrics provides monitoring of the GitHub related metrics.
+//
+// This depends on the metrics exporter of kubebuilder.
+// See https://book.kubebuilder.io/reference/metrics.html for details.
+package metrics
+
+import (
+	"net/http"
+	"strconv"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"sigs.k8s.io/controller-runtime/pkg/metrics"
+)
+
+func init() {
+	metrics.Registry.MustRegister(metricRateLimit, metricRateLimitRemaining)
+}
+
+var (
+	// https://docs.github.com/en/rest/overview/resources-in-the-rest-api#rate-limiting
+	metricRateLimit = prometheus.NewGauge(
+		prometheus.GaugeOpts{
+			Name: "github_rate_limit",
+			Help: "The maximum number of requests you're permitted to make per hour",
+		},
+	)
+	metricRateLimitRemaining = prometheus.NewGauge(
+		prometheus.GaugeOpts{
+			Name: "github_rate_limit_remaining",
+			Help: "The number of requests remaining in the current rate limit window",
+		},
+	)
+)
+
+const (
+	// https://docs.github.com/en/rest/overview/resources-in-the-rest-api#rate-limiting
+	headerRateLimit          = "X-RateLimit-Limit"
+	headerRateLimitRemaining = "X-RateLimit-Remaining"
+)
+
+// Transport wraps a transport with metrics monitoring
+type Transport struct {
+	Transport http.RoundTripper
+}
+
+func (t Transport) RoundTrip(req *http.Request) (*http.Response, error) {
+	resp, err := t.Transport.RoundTrip(req)
+	if resp != nil {
+		parseResponse(resp)
+	}
+	return resp, err
+}
+
+func parseResponse(resp *http.Response) {
+	rateLimit, err := strconv.Atoi(resp.Header.Get(headerRateLimit))
+	if err == nil {
+		metricRateLimit.Set(float64(rateLimit))
+	}
+	rateLimitRemaining, err := strconv.Atoi(resp.Header.Get(headerRateLimitRemaining))
+	if err == nil {
+		metricRateLimitRemaining.Set(float64(rateLimitRemaining))
+	}
+}