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; 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.