remove stage instances from prometheus
remove stage instances from prometheus

Bug: skia:
Change-Id: Id9b62614f07d2966db8cea206565e750986a515d
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/204766
Reviewed-by: Stephan Altmueller <43343ec95731c0e8daa7359daf7f2f3935582b24@google.com>
Commit-Queue: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@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