Stop using removed prometheus.MustRegisterOrGet

[fgrosse]

prometheus.MustRegisterOrGet is no longer available on the most recent master of https://github.com/prometheus/client_golang so the cmhttp package doesnt compile if that version is used (e.g. on a fresh go get github.com/classmarkets/cmhttp). This PR changes the code to use prometheus.MustRegister instead. Note that this breaks BC since it does no longer allow registering the same metric twice. We can probably find a way around this but I didnt want to invest to much time now since I do not use this decorator.

In hindsigt I think it was a mistake of adding the dependency to prometheus to this package. If you do not mind breaking BC then you could consider moving cmhttp.Instrumented and friends into a sub package.

[pschultz]

How about promoting reqCnt, reqDur, reqSz and resSz to the package level, initializing them in the first call to InstrumentedWithOpts, and re-using them on subsequent calls? Then we maintain the current semantics.

package cmhttp

var metrics struct {
    reqCnt *prometheus.CounterVec
    reqDur prometheus.Summary
    // ...
}

[fgrosse]

Sorry for the long delay: I somehow forgot about this open MR 😓

[pschultz]

Thanks!