Avoid duplicate vhost label for Prometheus queue-exchange metrics

[LoisSotoLopez]

Proposed Changes

Addresses #12347

Adds a specific clause on the
prometheus_rabbitmq_core_metrics_collector:labels function when the associated metric item is a Queue + Exchange combo ({Queue, Exchange}).

Types of Changes

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
• If relevant, I have added necessary documentation to https://github.com/rabbitmq/rabbitmq-website
• If relevant, I have added this change to the first version(s) in release-notes that I expect to introduce it

Further Comments

.

[LoisSotoLopez]

If provided implementation (supporting {Queue, Exchange}) looks good enough I could add a clause for when the Queue and Exchange are in reverse order ({Exchange, Queue}).

label({
    #resource{kind=exchange, name=ExName}
    #resource{kind=queue, virtual_host=VHost, name=QName},
 }) ->
    <<"vhost=\"", (escape_label_value(VHost))/binary, "\",",
      "exchange=\"", (escape_label_value(ExName))/binary, "\",",
      "queue=\"", (escape_label_value(QName))/binary, "\"">>;

[michaelklishin]

@LoisSotoLopez please do.

[michaelklishin]

This does address #12347. Once another function head is added, I will submit this branch so that Actions can run with access to secrets.

[LoisSotoLopez]

Preferred to add a comment similar to the one in

rabbitmq-server/deps/rabbitmq_prometheus/src/collectors/prometheus_rabbitmq_core_metrics_collector.erl

Line 479 in 8377eda

%% channel_queue_exchange_metrics {channel_id, {queue_id, exchange_id}}

, instead of adding unreachable code.

[michaelklishin]

@LoisSotoLopez thank you for contributing this.