Add new HTTPServer to serve /metrics on port 8082

This commit runs a new HTTPServer on port 8082 in a goroutine to serve
/metrics endpoint on a different port.

This also update the configurations and compose files.

Fixes: #1081

Signed-off-by: Vivek Singh <vivekkmr45@yahoo.in>

docker-compose.arm64.yml

@@ -3,6 +3,7 @@ services:
     gateway:
         ports:
             - 8080:8080
+            - 8082:8082
         image: openfaas/gateway:0.11.0-arm64
         networks:
             - functions