Per the latest review of I8031af03a9f229fa7f7577344291647bff25635e, the
dependency of the Metrics Platform on the Event Platform should be
clarified. We do so by:

* Introducing a generic EventSubmitter interface, implementations of
  which are responsible for submitting events to event ingestion
  services; and

* Making MetricsClient accept an instance of EventSubmitter and make
  MetricsClient#submit() defer to it

Supporting changes:

* Remove Integration#getHostName() as it is defunct

Bug: T281762
Change-Id: I23ae14a43e0bf440c22cd3980b0a2b32c1940414