Prometheus: some per-exchange/per-queue metrics aggregated per-channel

[gomoripeti]

(this is @LoisSotoLopez 's work, submitting while he is away)

Mostly identical to #11559 with tests fixed, and some minor renaming and reordering.

Proposed Changes

Add copies of some per-object metrics that are labeled per-channel aggregated to reduce cardinality. These metrics are valuable and easier to process if exposed on per-exchange and per-queue basis.

Implements #11217

TODO: document new metric groups

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Further Comments