Make common.InitWithMetrics use metrics.DefaultRegistry

BUG=skia:

Review URL: https://codereview.chromium.org/736093002
Fix skfiddle probers to work against new POST body and response format.

BUG=skia:

Review URL: https://codereview.chromium.org/739273003
Add alerts for the probers on skfiddle.

BUG=skia:

Review URL: https://codereview.chromium.org/741713003
This adds UI to select from the set of parameters and their values. It also extends the backend query code to work for the overview page (counts of labeled digests).

BUG=skia:2930

Review URL: https://codereview.chromium.org/736083002