Add Prometheus metrics to HTTP servers

We already have Prometheus metrics for HTTP clients (e.g., AWS SDK,
Pushgateway), but don't have it for servers. This has so far not been a
big issue because we tend to use gRPC more frequently, but now that we
have a more advanced authentication framework for HTTP, it makes sense
to give some more insight there as well.

pkg/global/apply_configuration.go

@@ -71,7 +71,11 @@ func (ls *LifecycleState) MarkReadyAndWait(group program.Group) {
 		}
 
 		group.Go(func(ctx context.Context, siblingsGroup, dependenciesGroup program.Group) error {
-			if err := bb_http.NewServersFromConfigurationAndServe(ls.config.HttpServers, router, group); err != nil {
+			if err := bb_http.NewServersFromConfigurationAndServe(
+				ls.config.HttpServers,
+				bb_http.NewMetricsHandler(router, "Diagnostics"),
+				group,
+			); err != nil {
 				return util.StatusWrap(err, "Failed to launch diagnostics HTTP server")
 			}
 			return nil

pkg/http/BUILD.bazel

@@ -11,6 +11,7 @@ go_library(
         "deny_authenticator.go",
         "header_adding_round_tripper.go",
         "jwt_authenticator.go",
+        "metrics_handler.go",
         "metrics_round_tripper.go",
         "oidc_authenticator.go",
         "server.go",

pkg/http/metrics_handler.go

@@ -0,0 +1,36 @@
+package http
+
+import (
+	"net/http"
+	"sync"
+
+	"github.com/buildbarn/bb-storage/pkg/util"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+var (
+	handlerPrometheusMetrics sync.Once
+
+	handlerRequestsDurationSeconds = prometheus.NewHistogramVec(
+		prometheus.HistogramOpts{
+			Namespace: "buildbarn",
+			Subsystem: "http",
+			Name:      "handler_requests_duration_seconds",
+			Help:      "Amount of time spent per HTTP request, in seconds.",
+			Buckets:   util.DecimalExponentialBuckets(-3, 6, 2),
+		},
+		[]string{"name", "code", "method"})
+)
+
+// NewMetricsHandler creates an adapter for http.Handler that adds basic
+// instrumentation in the form of Prometheus metrics.
+func NewMetricsHandler(base http.Handler, name string) http.Handler {
+	handlerPrometheusMetrics.Do(func() {
+		prometheus.MustRegister(handlerRequestsDurationSeconds)
+	})
+
+	return promhttp.InstrumentHandlerDuration(
+		handlerRequestsDurationSeconds.MustCurryWith(prometheus.Labels{"name": name}),
+		base)
+}