HPCC-31299 Publish metrics for the index cache

[richardkchapman]

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Testing:

Compiled

[github-actions[bot]]

Jira Issue: https://hpccsystems.atlassian.net//browse/HPCC-31299

Jirabot Action Result: Workflow Transition: Merge Pending Updated PR Changing assignee from: [email protected] to: [email protected]

[kenrowland]

I highly recommend that the clear is removed from the counter metric. Collection systems do not expect counters to be reset except in the case of a system restart. Based on comments from @jakesmith if the need persists to have the ability to clear the count of a counter metric, I think a new metric should be created. ClearableCounter, for example. That way it is explicit that it can be cleared, sinks can handle it as needed, and regular counters would not be cleared incorrectly by consumers of the metrics framework. I do stress that this new metric class should only be added if the need to clear the count persists.

[richardkchapman]

Pushed new commits with comments above addressed

[richardkchapman]

@ghalliday Squashed and rebased to latest 9.6.x