Add support for promhttp

[jeffreyyong]

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?

[jeffreyyong]

Manage to make the Go programme compile, but when it's done like this:

m := martini.New()
m.Use(promhttp.InstrumentHandlerInFlight(metrics.RequestGauge, m))

The whole programme just gets stuck

[zoidyzoidzoid]

I was recently looking at adding opencensus instrumentation to orchestrator which uses Martini too. It suggests doing it passing in a custom http.Handler too here

Though I'm looking at writing my own middleware for it instead.