NMS-13467: support additonal flow summary accumulation

[swachter]

Issue: https://issues.opennms.org/browse/NMS-13467

This PR

• adds the PaneAccumulator class that allows for an additional accumulation of result panes; if the commandline argument --summaryAccumulationDelayMs is non-zero then a PaneAccumulator is inserted into the pipeline

This PR contains some other minor commits that are a byproduct of the main work:

• The Benchmark application got some changes:
  • Cortex samples of different benchmark runs are differentiated by an additional label; this prevents "data conflicts" between different runs
  • The benchmark application now terminates properly in case of execution exceptions
  • A one-off error was fixed
• CortexIo got some changes:
  • additional counter metrics are provided that help to differentiate different kinds of Cortex response errors
  • the orgId configuration did not work
  • The implementation could be simplified by using Beam's @Key annotation
• Pipeline got a small change:
  • The pane- prefix was removed from pane label values