Support MetricsPusher in Dataflow Runner

See [relevant email thread](https://lists.apache.org/thread.html/2e87f0adcdf8d42317765f298e3e6fdba72917a72d4a12e71e67e4b5@%3Cdev.beam.apache.org%3E). From [~echauchot]:
 
_AFAIK Dataflow being a cloud hosted engine, the related runner is very different from the others. It just submits a job to the cloud hosted engine. So, no access to metrics container etc... from the runner. So I think that the MetricsPusher (component responsible for merging metrics and pushing them to a sink backend) must not be instanciated in DataflowRunner otherwise it would be more a client (driver) piece of code and we will lose all the interest of being close to the execution engine (among other things instrumentation of the execution of the pipelines).  I think that the MetricsPusher needs to be instanciated in the actual Dataflow engine._
 
 

Imported from Jira [BEAM-3926](https://issues.apache.org/jira/browse/BEAM-3926). Original Jira may contain additional context.
Reported by: swegner.
Subtask of issue #18594

Start by reading the Dataflow Runner and MetricsPusher entry points, then consult the linked email thread and Jira issue for the architectural context. Done means Dataflow Runner support is aligned with the intended MetricsPusher location and the relevant runner behavior is covered by tests, though no test file is named here.

Written by the indexing model from the issue text.