gorilla mux path compatibility

[caevv]

Paths are not supported with std.HandlerProvider

http_request_duration_seconds_count{code="500",handler="/hello/123",method="GET",service=""} 1
http_request_duration_seconds_count{code="500",handler="/hello/321",method="GET",service=""} 1

Should be:

http_request_duration_seconds_count{code="500",handler="/hello/{key}",method="GET",service=""} 2

Code example:

r := mux.NewRouter()

r.Use(std.HandlerProvider("", metrics_middleware.New(metrics_middleware.Config{
	Recorder: metrics.NewRecorder(metrics.Config{}),
})))

r.HandleFunc("/hello/{key}", hello).Methods("GET")