SuperManifestRefUpdatedListener: Make commit latency timer cumulative

The supermanifest/superproject_commit_latency timer was created with a
Description that is neither cumulative nor has a unit. A Gerrit server
binds MetricMaker to DropWizardMetricMaker, which rejects such timers,
so constructing SuperManifestRefUpdatedListener fails and the plugin
is not loaded:

  Cannot load plugin supermanifest
  com.google.inject.ProvisionException: Unable to provision ...
  Caused by: java.lang.IllegalArgumentException: timer must be cumulative
    at ...SuperManifestRefUpdatedListener.<init>(SuperManifestRefUpdatedListener.java:149)

The acceptance tests don't catch this because their in-memory server
binds MetricMaker to TestMetricMaker, which doesn't validate metric
descriptions.

Mark the timer cumulative and give it the milliseconds unit, as
DropWizardMetricMaker requires for timers.

Change-Id: Ie22f661b3b7522b68145125705299253c3164f1e