refactor: add MetricsBackend trait with dynamic metric registration

[tolulopedd26]

Summary

Refactors MetricsRegistry to support dynamic metric injection and custom labels via a new MetricsBackend trait.

Changes

• Adds MetricsBackend trait with register_counter(), register_gauge(), register_histogram() methods
• Implements dynamic metric storage with RwLock-protected HashMaps
• All 9 trait methods implemented: counter_inc, counter_inc_by, gauge_set, gauge_inc, gauge_dec, histogram_observe
• 4 new unit tests verifying registration, recording, and error handling

Closes #59