Add gateway_service_count metric for external provider

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

gateway/metrics/externalwatcher.go

@@ -0,0 +1,72 @@
+// Copyright (c) Alex Ellis 2017. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+package metrics
+
+import (
+	"encoding/json"
+	"io/ioutil"
+	"net"
+	"net/http"
+	"net/url"
+	"time"
+
+	"log"
+
+	"github.com/alexellis/faas/gateway/requests"
+)
+
+func AttachExternalWatcher(endpointURL url.URL, metricsOptions MetricOptions, label string, interval time.Duration) {
+	ticker := time.NewTicker(interval)
+	quit := make(chan struct{})
+	proxyClient := http.Client{
+		Transport: &http.Transport{
+			Proxy: http.ProxyFromEnvironment,
+			DialContext: (&net.Dialer{
+				Timeout:   3 * time.Second,
+				KeepAlive: 0,
+			}).DialContext,
+			MaxIdleConns:          1,
+			DisableKeepAlives:     true,
+			IdleConnTimeout:       120 * time.Millisecond,
+			ExpectContinueTimeout: 1500 * time.Millisecond,
+		},
+	}
+
+	go func() {
+		for {
+			select {
+			case <-ticker.C:
+
+				get, _ := http.NewRequest(http.MethodGet, endpointURL.String()+"system/functions", nil)
+
+				services := []requests.Function{}
+				res, err := proxyClient.Do(get)
+				if err != nil {
+					log.Println(err)
+					continue
+				}
+				bytesOut, readErr := ioutil.ReadAll(res.Body)
+				if readErr != nil {
+					log.Println(err)
+					continue
+				}
+				unmarshalErr := json.Unmarshal(bytesOut, &services)
+				if unmarshalErr != nil {
+					log.Println(err)
+					continue
+				}
+
+				for _, service := range services {
+					metricsOptions.ServiceReplicasCounter.
+						WithLabelValues(service.Name).
+						Set(float64(service.Replicas))
+				}
+
+				break
+			case <-quit:
+				return
+			}
+		}
+	}()
+}

gateway/server.go

@@ -77,6 +77,8 @@ func main() {
 		faasHandlers.DeployFunction = makeHandler(reverseProxy, &metricsOptions)
 		faasHandlers.DeleteFunction = makeHandler(reverseProxy, &metricsOptions)
 
+		metrics.AttachExternalWatcher(*config.FunctionsProviderURL, metricsOptions, "func", time.Second*5)
+
 	} else {
 		maxRestarts := uint64(5)