Prometheus: stream HTTP responses (a double digit % reduction of both CPU and memory footprint) (backport #14885) (backport #15018)

[mergify]

prometheus_text_format:format/1 produces a binary of the format for the entire registry. For clusters with many resources, this can lead to large replies from /metrics/[:registry] especially for large registries like per-object. Instead of formatting the response and then sending it, we can stream the response by taking advantage of the new format_into/3 callback (which needs to be added upstream to the prometheus dep). This uses cowboy_req:stream_body/3 to stream the iodata as prometheus works through the registry.

This should hopefully be a nice memory improvement. The other benefit is that results are sent eagerly. For a stress-testing example,

1. make run-broker
2. rabbitmqctl import_definitions path/to/100k-classic-queues.json
3. curl -s localhost:15692/metrics/per-object

Before this change curl would wait for around 8 seconds and then the entire response would arrive. With this change the results start streaming in immediately.

Discussed in #14865

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This is an automatic backport of pull request #14885 done by Mergify.

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This is an automatic backport of pull request #15018 done by Mergify.

[mergify]

Cherry-pick of 8e15d15 has failed:

On branch mergify/bp/v4.1.x/pr-15018
Your branch is up to date with 'origin/v4.1.x'.

You are currently cherry-picking commit 8e15d152b.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   deps/rabbitmq_prometheus/src/rabbit_prometheus_handler.erl

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   rabbitmq-components.mk

Cherry-pick of d6dd60e has failed:

On branch mergify/bp/v4.1.x/pr-15018
Your branch is ahead of 'origin/v4.1.x' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit d6dd60e8b.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   deps/rabbitmq_prometheus/Makefile
	modified:   deps/rabbitmq_prometheus/src/rabbit_prometheus_handler.erl
	modified:   deps/rabbitmq_prometheus/test/rabbit_prometheus_http_SUITE.erl

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   rabbitmq-components.mk

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[michaelklishin]

Relevant dependency series have changed between 4.1 and 4.2, 4.3 so this is more risky than it seemed at first. Fair enough, this change will be available in the 4.2.x series anyway.