Add Prometheus metrics endpoint

Add the Prometheus http.Handler to serve metrics at MetricsPath ("/metrics"
by default). This allows Prometheus to scrape metrics from OAuth2 Proxy.

Add a new middleware NewRequestMetrics and attach it to the preAuth
chain. This will collect metrics on all requests made to OAuth2 Proxy

Collapse some calls to Prinf() and os.Exit(1) to Fatalf as they are
equivalent. main() has a strict 50 lines limit so brevity in these
calls appreciated

main.go

@@ -1,8 +1,10 @@
 package main
 
 import (
+	"context"
 	"fmt"
 	"math/rand"
+	"net/http"
 	"os"
 	"os/signal"
 	"runtime"
@@ -12,6 +14,7 @@ import (
 	"github.com/ghodss/yaml"
 	"github.com/oauth2-proxy/oauth2-proxy/v7/pkg/apis/options"
 	"github.com/oauth2-proxy/oauth2-proxy/v7/pkg/logger"
+	"github.com/oauth2-proxy/oauth2-proxy/v7/pkg/middleware"
 	"github.com/oauth2-proxy/oauth2-proxy/v7/pkg/validation"
 	"github.com/spf13/pflag"
 )
@@ -42,8 +45,7 @@ func main() {
 
 	opts, err := loadConfiguration(*config, *alphaConfig, configFlagSet, os.Args[1:])
 	if err != nil {
-		logger.Printf("ERROR: %v", err)
-		os.Exit(1)
+		logger.Fatalf("ERROR: %v", err)
 	}
 
 	if *convertConfig {
@@ -53,36 +55,68 @@ func main() {
 		return
 	}
 
-	err = validation.Validate(opts)
-	if err != nil {
-		logger.Printf("%s", err)
-		os.Exit(1)
+	if err = validation.Validate(opts); err != nil {
+		logger.Fatalf("%s", err)
 	}
 
 	validator := NewValidator(opts.EmailDomains, opts.AuthenticatedEmailsFile)
 	oauthproxy, err := NewOAuthProxy(opts, validator)
 	if err != nil {
-		logger.Errorf("ERROR: Failed to initialise OAuth2 Proxy: %v", err)
-		os.Exit(1)
+		logger.Fatalf("ERROR: Failed to initialise OAuth2 Proxy: %v", err)
 	}
 
 	rand.Seed(time.Now().UnixNano())
 
+	oauthProxyStop := make(chan struct{}, 1)
+	metricsStop := startMetricsServer(opts.MetricsAddress, oauthProxyStop)
+
 	s := &Server{
 		Handler: oauthproxy,
 		Opts:    opts,
-		stop:    make(chan struct{}, 1),
+		stop:    oauthProxyStop,
 	}
 	// Observe signals in background goroutine.
 	go func() {
 		sigint := make(chan os.Signal, 1)
 		signal.Notify(sigint, os.Interrupt, syscall.SIGTERM)
 		<-sigint
-		s.stop <- struct{}{} // notify having caught signal
+		s.stop <- struct{}{} // notify having caught signal stop oauthproxy
+		close(metricsStop)   // and the metrics endpoint
 	}()
 	s.ListenAndServe()
 }
 
+// startMetricsServer will start the metrics server on the specified address.
+// It always return a channel to signal stop even when it does not run.
+func startMetricsServer(address string, oauthProxyStop chan struct{}) chan struct{} {
+	stop := make(chan struct{}, 1)
+
+	// Attempt to setup the metrics endpoint if we have an address
+	if address != "" {
+		s := &http.Server{Addr: address, Handler: middleware.DefaultMetricsHandler}
+		go func() {
+			// ListenAndServe always returns a non-nil error. After Shutdown or
+			// Close, the returned error is ErrServerClosed
+			if err := s.ListenAndServe(); err != http.ErrServerClosed {
+				logger.Println(err)
+				// Stop the metrics shutdown go routine
+				close(stop)
+				// Stop the oauthproxy server, we have encounter an unexpected error
+				close(oauthProxyStop)
+			}
+		}()
+
+		go func() {
+			<-stop
+			if err := s.Shutdown(context.Background()); err != nil {
+				logger.Print(err)
+			}
+		}()
+	}
+
+	return stop
+}
+
 // loadConfiguration will load in the user's configuration.
 // It will either load the alpha configuration (if alphaConfig is given)
 // or the legacy configuration.