LatencyMetric scope should include operator names

As of FLINK-7608 the latency metric scope contains the operator ID and subtask index of both the latency marker origin and recipient. We  do not however expose the operator names.

This is inconsistent with existing metrics about operators (although this is a big can of worms since various task-level fields like executionId are also missing), but more importantly does not allow for any stable identification across job submissions, as the IDs always change.