[datahopper] Actually start task metrics
[datahopper] Actually start task metrics

Forgot to add this in the last CL.

Bug: skia:
Change-Id: I17272b4888bbf0b58186958bc5e9a670022e6399
Reviewed-on: https://skia-review.googlesource.com/c/171567
Reviewed-by: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Reviewed-by: Ben Wagner <3ef7217be91069877d94f7907ce5479000772cd3@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