Prevent non-zero publisher count in Grafana when aggregating metrics

[coro]

In the case where there are 0 channels (and as such 0 publishers), the
dashboard reports there are actually n publishers in an n-node
cluster.

Proposed Changes

This changes the calculation of publishers to be number of
channels (which is always known) minus the number of consumers (which is
always known). This avoids the issue where there is ambiguity where a channel
which is publishing, but has not actively published yet.

Prior to this change, a cluster configured with prometheus.return_per_object_metrics = false will always report a non-zero number of publishers, even if there is no traffic on that cluster. For my example screenshots, I am using a three-node cluster with a random RabbitMQ pod being killed every minute, over the course of 5 minutes. You can see that there are always n publishers reported in Grafana for n nodes:

After this change, this is correctly reported as 0:

I confirmed that the metric behaves as usual where there are non-zero publishers using PerfTest with 5 consumers & publishers:

Types of Changes

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• 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)
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Further Comments

Discovered this with @gerhard while we were working on rabbitmq/tgir#19

[gerhard]

Works as expected, thanks! We need to update the dashboard json to the one generated by Grafana 7 before we can upload this to grafana.com. Do this post-merge.

The failing tests are not related to this since no source has changed. We need to drop OTP v21.3 support, Bump Elixir to >= 1.10 & add OTP v23.1 support. This will happen part of the monorepo migration, no point in fixing these GitHub Actions.