Move probers from using metrics.Timer to using metrics.Gauge.
Move probers from using metrics.Timer to using metrics.Gauge.

Once this lands I'll send a second CL that updates all the alerts on prober metrics.

BUG=skia:

Review URL: https://codereview.chromium.org/1111393002

Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot