Add debug logging to datahopper.
Add debug logging to datahopper.

We've seen datahopper stop generating metrics every evening lately. I'm adding some debug logging temporarily to help locate the cause of the hang.

BUG=skia:5564

Review-Url: https://codereview.chromium.org/2174953002

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot