[18-stable] Generate and configure Prometheus client certs

[vyzigold]

When TLS and metricStorage are enabled, generate client certificate for Prometheus. This will later be used by Prometheus to access metric exporters which require client certificates.

jira: https://redhat.atlassian.net/browse/OSPRH-33208
Depends-On: openstack-k8s-operators/telemetry-operator#976
Generated-By: Claude-Code claude-opus-4-6

Cherry-pick-of: #2014

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[vyzigold]

Requires openstack-k8s-operators/telemetry-operator#1020 to merge first and telemetry-operator update here.