## What is the purpose of the change

This PR fixes a severe issue in the metric system where chained batch operators 
would always operate on the same `OperatorMetricGroup`. As a result most 
Flink-provided metrics were not exposed for chained operators at all, while 
other metrics, like task-level IO metrics, were render incorrect.

The problem is that we used the tasks `VertexID` to identify operators; which 
is obviously identical for all operators in a chain. We now use the vertexID 
and operator name to identify them.

## Brief change log

* fix identification in `TaskMetricGroup` by using both the ID and operator name
* extend `MockEnvironment[Builder]` to allow the `TaskMetricGroup` to be set

## Verifying this change

This change added tests:
* ChainedOperatorsMetricTest
* run a basic wordcount as described in the JIRA and verify the results via the 
UI/reporter of your choice


[ Full content available at: https://github.com/apache/flink/pull/6599 ]
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