Support MetricsPusher in Direct Runner

There is some users that use DirectRunner as a production runner for very small scale. So it becomes a target for the Metrics Pusher also. There is a default java implementation of the Metrics Pusher in runner-java-core in this PR [https://github.com/apache/beam/pull/4548](https://github.com/apache/beam/pull/4548.); it can be used or not by the java runners. But it relies on `MetricsContainerStepMap` that direct runner does not use. The ongoing work on portability of the `MetricsPusher` will provide a langage agnostic representation of `MetricsPusher.` Direct Runner will be free to read the representation and instantiate a MetricsPusher of its own.

Imported from Jira [BEAM-3957](https://issues.apache.org/jira/browse/BEAM-3957). Original Jira may contain additional context.
Reported by: echauchot.
Subtask of issue #18594

Start by reading the Direct Runner metrics path and the portability work for MetricsPusher. Compare its available metrics representation with the runner-java-core implementation and its reliance on MetricsContainerStepMap. Done means Direct Runner can instantiate and use a MetricsPusher without that unsupported container map.

Written by the indexing model from the issue text.