Improve step names in portable runners

Step names are currently inconsistent between runners. This is probably unavoidable due to fusion, but we should design a system that is more consistent between runners so that most metric queries against the Fn API runner can also be used with the portable Flink and Spark runners.

Spark Runner:
MetricKey(step=ref_AppliedPTransform_count1_17, metric=MetricName(namespace=ns, name=counter), labels={}): 2
MetricKey(step=ref_AppliedPTransform_count2_18, metric=MetricName(namespace=ns, name=counter), labels={}): 4
...

Fn API Runner:
MetricKey(step=count1, metric=MetricName(namespace=ns, name=counter), labels={}): 2,
MetricKey(step=count2, metric=MetricName(namespace=ns, name=counter), labels={}): 4

Imported from Jira [BEAM-9997](https://issues.apache.org/jira/browse/BEAM-9997). Original Jira may contain additional context.
Reported by: ibzib.