This wasn't being recorded correctly because the clientId is only
set in LoginFlow after the flow succeeds, whereas we were
previously logging the clientId before executing the flow. This
adds special handling for LoginFlow. Note that we only set the
metric label to the clientId for valid registrar logins, to ensure
that metric cardinality doesn't grow unbounded (as it might if we
used every arbitrary string passed in as an attempted login).
This also refactors creation and handling of FakesAndMocksModule so
as to be able to make test assertions about EPP metrics from
integration flow tests.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=154048280
