[stable/postgresql] Fix upgradability of metrics deployment.

[desaintmartin]

See #7726.
Current state prevents to upgrade the whole chart if metrics are used.

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• DCO signed
• Chart Version bumped
• Variables are documented in the README.md

[k8s-ci-robot]

Hi @desaintmartin. Thanks for your PR.

I'm waiting for a helm member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[davidkarlsen]

/ok-to-test
/lgtm
/assign

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: davidkarlsen, desaintmartin

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