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
I would agree about premature optimization in general - but why explicitly add code that does more than needed, and whose results are just thrown away? Or do we really have a need to export all the individual filedescriptors at some point?

(seems like this method is now not called at all, while the new FileDescriptorsLen one does more than what it's used for - maybe I'm missing some future plans?)