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

pkg/apis/options/options.go

@@ -22,6 +22,7 @@ type Options struct {
 	ProxyPrefix        string   `flag:"proxy-prefix" cfg:"proxy_prefix"`
 	PingPath           string   `flag:"ping-path" cfg:"ping_path"`
 	PingUserAgent      string   `flag:"ping-user-agent" cfg:"ping_user_agent"`
+	MetricsAddress     string   `flag:"metrics-address" cfg:"metrics_address"`
 	HTTPAddress        string   `flag:"http-address" cfg:"http_address"`
 	HTTPSAddress       string   `flag:"https-address" cfg:"https_address"`
 	ReverseProxy       bool     `flag:"reverse-proxy" cfg:"reverse_proxy"`
@@ -132,14 +133,14 @@ func (o *Options) SetRealClientIPParser(s ipapi.RealClientIPParser) { o.realClie
 // NewOptions constructs a new Options with defaulted values
 func NewOptions() *Options {
 	return &Options{
-		ProxyPrefix:        "/oauth2",
-		ProviderType:       "google",
-		PingPath:           "/ping",
-		HTTPAddress:        "127.0.0.1:4180",
-		HTTPSAddress:       ":443",
-		RealClientIPHeader: "X-Real-IP",
-		ForceHTTPS:         false,
-
+		ProxyPrefix:                      "/oauth2",
+		ProviderType:                     "google",
+		MetricsAddress:                   "",
+		PingPath:                         "/ping",
+		HTTPAddress:                      "127.0.0.1:4180",
+		HTTPSAddress:                     ":443",
+		RealClientIPHeader:               "X-Real-IP",
+		ForceHTTPS:                       false,
 		Cookie:                           cookieDefaults(),
 		Session:                          sessionOptionsDefaults(),
 		Templates:                        templatesDefaults(),
@@ -201,6 +202,7 @@ func NewFlagSet() *pflag.FlagSet {
 	flagSet.String("proxy-prefix", "/oauth2", "the url root path that this proxy should be nested under (e.g. /<oauth2>/sign_in)")
 	flagSet.String("ping-path", "/ping", "the ping endpoint that can be used for basic health checks")
 	flagSet.String("ping-user-agent", "", "special User-Agent that will be used for basic health checks")
+	flagSet.String("metrics-address", "", "the address /metrics will be served on (e.g. \":9100\")")
 	flagSet.String("session-store-type", "cookie", "the session storage provider to use")
 	flagSet.Bool("session-cookie-minimal", false, "strip OAuth tokens from cookie session stores if they aren't needed (cookie session store only)")
 	flagSet.String("redis-connection-url", "", "URL of redis server for redis session storage (eg: redis://HOST[:PORT])")