Refactoring prometheus metrics and labels (#2375)

This change mainly fixes few things around router and executor
exposed metrices.
1. We are trying to follow standard in metric names.
2. Lables such as namespace are colliding with kube-prometheus standards
so they are getting relabled to exported_namespace. Added function prefix
to resolve this.

Signed-off-by: Sanket Sudake <sanketsudake@gmail.com>

pkg/router/metrics.go

@@ -43,12 +43,12 @@ var (
 	metricAddr = ":8080"
 
 	// function + http labels as strings
-	labelsStrings = []string{"cached", "namespace", "name", "host", "path", "method", "code"}
+	labelsStrings = []string{"cached", "function_namespace", "function_name", "host", "path", "method", "code"}
 
 	// Function http calls count
 	// cached: true | false, is this function service address cached locally
-	// namespace: function namespace
-	// name: function name
+	// function_namespace: function namespace
+	// function_name: function name
 	// code: http status code
 	// path: the client call the function on which http path
 	// method: the function's http method