Move off of m3db prometheus client libs

M3 depends on mid-2018 forks of the various prometheus/client_golang -related projects (which appear to have no modifications). The principle reason is to make it so M3, and projects dependent on M3, are able to stay near "latest-stable" of this important library, for all the normal reasons that's desirable.

Also of note, tally depends on these "m3db prom" forks. This pending PR to the (new) chronosphere tally fork moves that project off these libs as well, and the M3 PR point to this fork.

+1 for this move.