Prometheus: add segment file count to queue_metrics and expose

[markusgust]

Proposed Changes

Off the back of discussion in a previous PR: #10275 (comment)

Adding segment count to queue_metrics ets table and expose value prometheus. Exposing the current segment count of a stream can help users calculate a rough stream size using segment counts * max-segment-size.

If merged, will add the necessary documentation to https://github.com/rabbitmq/rabbitmq-website in a separate PR.

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[kjnilsson]

I think this looks fine as it is only exposing a pre-existing counter value.

Ideally I think we should maintain a gauge in osiris with a more accurate stream size (including indexes) and expose that through osiris_counters but that can be added later.