Standard exports

The procfs package can be used to query the pseudo-filesystem commonly
mounted under /proc. It currently provides information from /proc/stat,
/proc/$pid/stat, /proc/$pid/fd and /proc/$pid/limits.

This change adds two new collectors to the prometheus package which
export metrics about a given or the current process.

* ProcessCollector exports metrics about cpu time, vss, rss, fd usage as
  well as the start time of a given process.
* GoCollector exports currently only the number of active goroutines.

Both collectors are active by default.

Background: https://docs.google.com/document/d/1Q0MXWdwp1mdXCzNRak6bW5LLVylVRXhdi7_21Sg15xQ/edit?usp=sharing
But we should not make that future PR overly complicated. I believe having that PidFn makes it difficult to later create a constant label for the pid. Is there a reason for not just passing in a static value? I guess we want it to be variable to track restarting processes with one and the same collector. Which makes me think we might want a 'symbolic' name for the label and not a numeric PID at all (or only as the default), so perhaps we need two different parameters here, a static string for the label value and a function as we have now for a dynamic numeric PID, with reasonable defaults in either case.

A different thought is that we want consistent label sets. If we don't add any kind of process-name of pid label right now, but do it later, we will have inconsistent label sets. So we might want to think a minute or two about the label right now.