Move from metric.Timer to metric.Gauge for last time since update metrics.
Move from metric.Timer to metric.Gauge for last time since update metrics.

The go-metrics package is to smart, use a simple gauge in seconds and do
the averaging, etc, via InfluxDB.

BUG=skia:

Review URL: https://codereview.chromium.org/713823004

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