Return labels in functions list endpoint

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

gateway/requests/requests.go

@@ -67,6 +67,10 @@ type Function struct {
 	InvocationCount float64 `json:"invocationCount"` // TODO: shouldn't this be int64?
 	Replicas        uint64  `json:"replicas"`
 	EnvProcess      string  `json:"envProcess"`
+
+	// Labels are metadata for functions which may be used by the
+	// back-end for making scheduling or routing decisions
+	Labels *map[string]string `json:"labels"`
 }
 
 // AsyncReport is the report from a function executed on a queue worker.