new class: FingerMetrics/Metrics

Metrics is a class to persistently store information on fingers and
derive common metrics from this information.
Each interpreter can get access to a Metrics class by setting
requires_metrics_ = true, if it cannot get Metrics from the previous
interpreter in the filter chain it will create one.
Note that some interpreter (scaling, lookahead) invalidate existing
metrics information and pass NULL to their next_ to have it create a new
instance if it needs one.
As properties can only be registered once, we require a separate
MetricsProperties class that has to be passed to the filter chain.

BUG=chromium:226699
TEST=check for regressions

Change-Id: I0a3bcd3fdb4b1706f86e3eedcddb726335b103c4
Reviewed-on: https://gerrit.chromium.org/gerrit/49539
Commit-Queue: Dennis Kempin <denniskempin@chromium.org>
Reviewed-by: Dennis Kempin <denniskempin@chromium.org>
Tested-by: Dennis Kempin <denniskempin@chromium.org>