Use recommended instance label for Prometheus/Alertmanager resources

[maxbrunet]

Description

This sets the recommmended app.kubernetes.io/instance label on all Prometheus/Alertmanager component resources, and removes the legacy prometheus/alertmanager ones.

Prometheus already had the instance label in selectorLabels, Alertmanager did not. Is there any concern about having it in commonLabels for both? I have tested these labels in my setup without issues.

For reference, the recommended labels were added in prometheus-operator/prometheus-operator#3841, this is in the continuity of #832

Type of change

• CHANGE (fix or feature that would cause existing functionality to not work as expected)
• FEATURE (non-breaking change which adds functionality)
• BUGFIX (non-breaking change which fixes an issue)
• ENHANCEMENT (non-breaking change which improves existing functionality)
• NONE (if none of the other choices apply. Example, tooling, build system, CI, docs, etc.)

Changelog entry

- Use recommended instance label for Prometheus/Alertmanager resources

[paulfantom]

Overall LGTM. Thanks for cleaning this up, now the code looks much cleaner!

One nit: let's mark this as a CHANGE in PR description as this is changing selectors and thus prevents clean upgrade.

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Is there any concern about having it in commonLabels for both?

This label should be in commonLabels in every component (prometheus, alertmanager, node-exporter, etc.).

selectorLabels should contain the same labels as commonLabels minus ones that mutate between upgrades (like version)

[maxbrunet]

One nit: let's mark this as a CHANGE in PR description as this is changing selectors and thus prevents clean upgrade.

I have updated the description, but this does not prevent clean upgrade as no label is added or removed from the pods and no immutable selector being updated here

[paulfantom]

no label is added or removed from the pods and no immutable selector being updated here

commonLabels are passed into selectorLabels and only app.kubernetes.io/version is excluded. This means that by adding app.kubernetes.io/instance to commonLabels this PR is changing all selector entries in all prometheus objects. In the end better safe than sorry :)