Move /healthz handler to handlers package

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

gateway/handlers/healthhandler.go

@@ -0,0 +1,20 @@
+// Copyright (c) OpenFaaS Author(s) 2019. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+package handlers
+
+import "net/http"
+
+//HealthzHandler healthz hanlder for mertics server
+func HealthzHandler(w http.ResponseWriter, r *http.Request) {
+
+	switch r.Method {
+	case http.MethodGet:
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte("OK"))
+		break
+
+	default:
+		w.WriteHeader(http.StatusMethodNotAllowed)
+	}
+}