[task scheduler] Add task-scheduler-staging to prometheus.yml
[task scheduler] Add task-scheduler-staging to prometheus.yml

Bug: skia:
Change-Id: Ib1bba2ec655eb254d8fc5d6bfd0b8e3cecc4123b
Reviewed-on: https://skia-review.googlesource.com/c/177121
Reviewed-by: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot