Smarter path label flooding avoidance

@diasdavid and I have a little service running that we instrument with epimetheus. There's two little issues:

• Every possible request coming from the internet is counted, e.g. path="/phpmyadmin" or path="/wp-login.php"
• We have only one route, which is at the root and parameterized (/{email}). Every possible parameter is counted, e.g. path="/david@example.com" and path="/lars@example.com".

I'd propose using request.route.path instead of the actual path, which would end up as e.g. a path="/{email}" label. If the route is undefined, we'd fall back to path="".

We're using HAPI here, and I'm not sure whether the other server libraries expose the route of a request.

[ashoksahoo]

I have a flooding of /callback/id and my Prometheus server went down.

[sijmenvos]

Agreed, for Restify I'm also getting multiple paths:

http_request_duration_milliseconds{quantile="0.95",method="get",path="/api/resource/1/",cardinality="one",status="200"} 187
http_request_duration_milliseconds{quantile="0.9",method="get",path="/api/resource/2/",cardinality="one",status="200"} 59
etc..

In Restify I've changed the path to request.route.path so it shows /api/resource/:id/

Like in pull request: #24