Report the per-job build wait time to graphite

Knowing how long a job waits to build in aggregate can give useful
insights into the performance and capacity of the build system. This
change also uses the node labels sent back from the gearman worker to
submit metrics within that context.

Change-Id: I173251c8b1569755124b7cb1a48b6274bf38c94b
Depends-On: Ibca938fcf8a65facd7e39dab4eb994dfc637722a
5 files changed