feat(prometheus): allow passing a *ServeMux on NewReporter

[mlcdf]

In the prometheus package, the NewReporter function exposes an HTTP server for the metrics. It's great, but if you also have other routes to exposes on your app, you need to create another HTTP server to handle those.

I propose adding a ServerMux option so that user of the lib can expose other endpoints on the same HTTP server started. I'm thinking of other observability related endpoints such as pprof (which is my very use case).

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