[Task]: Add a first-class pipeline option for `--experiments=enable_stackdriver_agent_metrics`

### What needs to happen?

A pipeline option for Dataflow needs to be added, and internally this needs to fill up the experiment.

Once this is done, we need to update our documentation. This will give the correct message that this is fully supported (https://cloud.google.com/dataflow/docs/guides/using-cloud-monitoring)

### Issue Priority

Priority: 2

### Issue Component

Component: runner-dataflow