add sync_agent_published_resource metric

[SimonTheLeg]

Summary

Adds a metric on how many published_resources are managed by the agent. Opposed to the initial issue, a gauge was chosen instead of a counter as this metric of course can decrease if a published ressource is delete.

The updating of the metric itself is done directly in the APIExport reconciliation loop as we do trigger this for all ApiExports as well as PublishedResouces with RessourceSchemas and we already to a list call there, which is perfect for determining the number of managed Published Ressources.

With regards to testing: I think we have two options here, both are not perfect. I have split them in separate commits. The unit one is kind of superficial, it just creates fakeclients and then asserts that the metric variable has been changed by the reconciler. Maybe this is good enough? Alternatively, we can have that in a proper e2e test, there the only difficulty was obtaining a free port to fetch the metrics from and it takes a lot longer.

As said, please let me know what you prefer.

What Type of PR Is This?

/kind feature

Related Issue(s)

Fixes #160

Release Notes

NONE

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[kcp-ci-bot]

@SimonTheLeg: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-api-syncagent-verify	038f77a	link	true	/test pull-api-syncagent-verify
pull-api-syncagent-test-e2e-kcp-0.29	038f77a	link	true	/test pull-api-syncagent-test-e2e-kcp-0.29

Full PR test history

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