[prom] Address Prometheus interface change
[prom] Address Prometheus interface change

Caused by https://github.com/prometheus/client_golang/commit/d01fd6222241828327e3507d64c90f796673bad0

Bug: skia:
Change-Id: I7d875c460089c9332d815ea5f4cd7d43305a30c6
Reviewed-on: https://skia-review.googlesource.com/14241
Commit-Queue: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>
Commit-Queue: Ben Wagner <3ef7217be91069877d94f7907ce5479000772cd3@google.com>
Reviewed-by: Ben Wagner <3ef7217be91069877d94f7907ce5479000772cd3@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