[GH#10755]: Clarify operator-managed Typha metrics service

[clcc2019]

Product Version(s):
Calico Open Source next, 3.29, 3.30, 3.31, and 3.32.

Issue:
Fixes projectcalico/calico#10755

Link to docs preview:
Pending the automatic Netlify preview build.

SME review:

• An SME has approved this change.

DOCS review:

• A member of the docs team has approved this change.

Additional information:

• Explains that setting spec.typhaMetricsPort makes the operator create calico-typha-metrics.
• Updates the Operator Prometheus scrape configuration to discover that managed service.
• Leaves the service lifecycle to the operator during cleanup.
• Keeps the manual-install instructions unchanged.

Validation:

• git diff --check
• Verified the service name and lifecycle against the operator render implementation.

Merge checklist:

• Deploy preview inspected wherever changes were made
• Build completed successfully
• Tests have passed

@tigera/docs please review when ready.

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