[autoroll] Add metric for last successful sheriff retrieval
[autoroll] Add metric for last successful sheriff retrieval

Also, log the current sheriff at every tick.

Bug: skia:10900
Change-Id: I6bc3dcde9320eac959c46fdc3ef6d8b45da6b484
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/331398
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Reviewed-by: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@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