Implement Custom Metrics Cardinality Guard

[maugauwi-hash]

Labels: bug, observability
Priority: Medium
Estimated Time: 1.5 hours

Description:
backend/src/custom_metrics.rs allows registering custom metrics, but unbounded label cardinality (e.g., using a user ID as a label) could blow up the metrics backend's memory and cost.

Tasks:

• Add a cardinality limit per metric label
• Reject or drop label values beyond the limit with a warning log
• Add tests for cardinality limit enforcement
• Document safe labeling practices in docs/custom-dashboards.md

[apatafamilycompound123-ops]

@apatafamilycompound123-ops has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello Maintainer, I’ve reviewed the issue and understand the requirements. I’m confident I can handle it effectively. Kindly assign it to me.

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