Add support for promhttp

`InstrumentHandlerResponseSize func(obs prometheus.ObserverVec, next http.Handler) http.Handler` from the `promhttp` package takes in "http.Handler" and returns a "http.Handler" for instrumenting the response size. However, martini only has `martini.Handler` which makes it impossible to pass into the promhttp instrumentation function, I'm wondering if it's possible to use martini with prometheus `promhttp`?