[release-5.8] Backport metrics and status handling from 5.9

[xperimental]

What this PR does / why we need it:

This PR combines multiple changes that were backported to 5.9 and backports them to 5.8:

• operator: Add warning for old schema configuration grafana/loki#11158 (only the status handling, not the warning itself)
• fix(operator): Keep credentialMode in status when updating schemas grafana/loki#12212 (reduced to not overwrite the complete status)
• feat(operator): Restructure LokiStack metrics grafana/loki#12228 (same implementation, removed tests for warning condition)

This should bring 5.8 in line with the newer releases regarding metrics and bring the improvements we made to the handling of the status conditions back to 5.8 as well.

Which issue(s) this PR fixes:

Fixes LOG-5857

[openshift-ci]

@xperimental: all tests passed!

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[periklis]

/lgtm

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: periklis, xperimental

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