test: add Kuadrant operator metrics tests

[silvi-t]

Description

• Add observability tests for Kuadrant operator health metrics (existence, readiness, components, dependencies, controllers)
• Add observability tests for Kuadrant policy metrics (kuadrant_policies_total and kuadrant_policies_enforced) across all policy kinds
• Add disruptive lifecycle test verifying policy metrics increment/decrement on policy create/delete
• Fix assertion message in existing test_observability.py for clarity

Closes #875

Verification steps

poetry run pytest -vv -n4 --dist loadfile\
  testsuite/tests/singlecluster/observability/test_kuadrant_operator_health_metrics.py \
  testsuite/tests/singlecluster/observability/test_kuadrant_policy_metrics.py

The lifecycle test requires sequential execution (marked disruptive):

poetry run pytest -vv testsuite/tests/singlecluster/observability/test_kuadrant_policy_metrics_lifecycle.py

[averevki]

Try to get all relevant metrics only once, from the shared fixture. Then use .filter() on the returned structure to filter metrics you got from a single request to jaeger. Check out the methodology other testsuite tests for metrics follow

[silvi-t]

I looked into the existing metrics tests and the .filter() approach isn't unified across the testsuite — some tests use has_label(), others use raw lambdas, and some don't use a shared fixture at all. I can refactor this tests to use the shared fixture + .filter()pattern.
Should I also refactor the other metric tests so that they use a uniform methodology, so that it will be unified throughout the test suite?

[averevki]

CoreDNS tests will probably have the best usage of the interface for metrics we have in the testsuite currently, I would inspire from them. The main point with filtering on already collected metrics is less API calls done from testsuite.

Should I also refactor the other metric tests so that they use a uniform methodology, so that it will be unified throughout the test suite?

that would be great, doesn't need to be in this PR though 👍

[silvi-t]

I've refactored the metrics tests based on the suggested CoreDNS pattern. I will also open a new issue to unify the rest of the metrics tests across the testsuite to follow the same approach, so we do not forget about that.