add method or to the context a way to retrieve the endpoint

[nbari]

When instrumenting endpoints with a wildcard *, instead of logging all the possible requests would be better to just use the name of the "endpoint", this would avoid having to do something like this:

 endpoint := "*"
 for _, v := range []string{"/foo/", "/foo"} {
       if r.URL.Path == v {
            endpoint = "/foo"
            break
       }
 }

[nbari]

implemented in version 5.0.0

The idea is to create / add names to routes like this:

 router.HandleFunc("/path", handler).Name("route-name")

To retrieve the name of the route in the requests:

 endpoint := violetear.GetRouteName(r)
 c.WithLabelValues(endpoint).Observe(time.Since(start).Seconds())