FR: Pending builds per builder, elapsed time for current build per builder

[delroth]

[I may end up implementing that myself in the future, but filing a FR since other people might be interested and might know the codebase better than I do.]

On dolphin-emu's buildbot we keep hitting "stuck builds" blocking our builders until someone takes manual action. It would be great if buildmaster was more resilient to random failures in general, but in the meantime having monitoring+alerting for stuck builds would improve MTTR quite a lot.

The two main metrics I would suggest to be added are:

• Count of pending/queued builds, per builder.
• Elapsed time for current build, per builder (or 0 if no build in progress).

WDYT?