Migrate pprof init and mage code to elastic-agent-libs

[fearful-symmetry]

This is a continuation of an issue here elastic/elastic-agent-shipper#6 (comment) that I discussed with @ph .

Basically, as we start work on https://github.com/elastic/elastic-agent-shipper, there's a great deal of overlapping functionality with agent, but a lot of that functionality isn't importable and/or lives in internal/ Part of that is the pprof HTTP init process, which (among other functions) is setupMetrics in internal/pkg/agent/cmd/run.go. There's also the various config objects in internal/pkg/core/monitoring/config/config.go. It would be relatively nice if we could make these more generic and move them into the elastic-agent-libs library, where we can then import them back into agent and the shipper.

[cmacknz]

If we want to move elastic/elastic-agent-shipper#6 forward with this path we'll have to do this ourselves.

I've split the mage portion into a separate issue (#321) which I'll take care of.

[jlind23]

@fearful-symmetry is this something you will take care of? Just asking as you are assigned to it.

[fearful-symmetry]

@jlind23 yah, I think I'm going to close this, most of these issues ended up being dealt with indirectly or by other people. We can open a different, more specific issue if there's another related part of this I've forgotten about.