What is missing?

While the operator prints warning logs when it notices usage of deprecated fields in custom resources, it's hard for users to notice it. In particular when they don't have access to the operator's logs (e.g. they may be able to provision Prometheus resources but the operator is managed by another team).

Suggested improvements:

• Report usage of deprecated fields in the Reconciled condition. The condition's status should be True but the reason and message fields can convey additional information (such as reason: DeprecatedFieldsInUse and message: Field xxx is deprecated and may be removed in a future version, please use yyy instead.).
• Report a gauge metric per resource signaling how many deprecated fields are used.
• Create an alert from the metric.

We also need to make sure that all deprecated fields are reported (for instance prometheusRulesExcludedFromEnforce is not).

Why do we need it?

Better feedback to users who are referencing fields that are being deprecated.

Environment

• Prometheus Operator version:

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Anything else we need to know?: