If a given pidFn for a process collector can't determine the pid, this
had to be signaled with an invalid pid so far (pid <= 0). In order to
make the error interface clearer for users, this change introduces an
explicit error parameter.
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.
Tobias Schmidt