Discard golang / process metrics about haproxy_exporter

[SuperQ]

The node_exporter does not expose metrics about other processes. These are not duplicates of what the node_exporter exposes.

[Gui13]

I understand it doesn't expose metrics from other processes, but the haproxy_exporter exposes metrics about its own internal go process.

I think this is the standard operation mode of the prometheus library, and I know this might be important to be sure that the collector process works correctly, but in production IMHO there is no need for the 30-or-so metrics added from the library.

I believe having a command line flag to suppress non-haproxy metrics from being exposed by the metrics endpoint would be a good idea. Would you be ok with me adding this as a PR?

[Gui13]

FWIW this is an older nitpick of people, and the solution is to create a specific registry to expose metrics apart from what the prom library internally collects: https://stackoverflow.com/questions/35117993/how-to-disable-go-collector-metrics-in-prometheus-client-golang

[SuperQ]

This can be done with metric relabel configs.