[task scheduler] Add metrics for free bot count.
[task scheduler] Add metrics for free bot count.

Change-Id: Ib73ac2c65eb6edd5e0abc8d2975d2c71eaa239a3
Reviewed-on: https://skia-review.googlesource.com/141266
Reviewed-by: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Commit-Queue: 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