Define API for getting non user metrics.

MetricResults API does not support a way to query system metrics. Different implementations accomplish the same thing in different ways.

FnApiRunner has a monitoring_metrics() defined on its PipelineResult (RunnerResult) object.
DataflowRunner, has a all_metrics() method defined on its MetricResults (DataflowMetrics) object.

Dataflow implementation of using metrics().all_metrics() sound reasonable. It would be usefult to define a single API that all runners implementations could agree on.

cc: [~pabloem] [~robertwb] [~Ardagan]

Imported from Jira [BEAM-8429](https://issues.apache.org/jira/browse/BEAM-8429). Original Jira may contain additional context.
Reported by: altay.