add completionCount constants for co-ordinator and shard level tasks

[kaushalmahi12]

Adding completionCount metrics constants. This was previously not part of /stats/search_backpressure API as the change got merged post 2.11 release. We will need this metric to find correct cancellation percentage with respect to total completed tasks at shard and co-ordinator level.

Github issue: https://github.com/opensearch-project/performance-analyzer/issues/587

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Check List

• [ ] New functionality includes testing.
  • [ ] All tests pass
• [ ] New functionality has been documented.
  • [ ] New functionality has javadoc added
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[ansjcy]

LGTM.

[deshsidd]

LGTM apart from the minor nit