Add RED metrics

- this PR adds metrics via Prometheus for instrumentation and
to move towards using HPAv2 / custom metrics in Kubernetes.

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

watchdog/main.go

@@ -19,6 +19,7 @@ import (
 	"syscall"
 	"time"
 
+	"github.com/openfaas/faas/watchdog/metrics"
 	"github.com/openfaas/faas/watchdog/types"
 )
 
@@ -58,12 +59,19 @@ func main() {
 		MaxHeaderBytes: 1 << 20, // Max header of 1MB
 	}
 
+	httpMetrics := metrics.NewHttp()
 	log.Printf("Read/write timeout: %s, %s. Port: %d\n", readTimeout, writeTimeout, config.port)
 	http.HandleFunc("/_/health", makeHealthHandler())
-	http.HandleFunc("/", makeRequestHandler(&config))
+	http.HandleFunc("/", metrics.InstrumentHandler(makeRequestHandler(&config), httpMetrics))
+
+	metricsServer := metrics.MetricsServer{}
+	metricsServer.Register(config.metricsPort)
+
+	cancel := make(chan bool)
+
+	go metricsServer.Serve(cancel)
 
 	shutdownTimeout := config.writeTimeout
-
 	listenUntilShutdown(shutdownTimeout, s, config.suppressLock)
 }