feat: integrate telemetry metrics

[Stanley-Owoh]

[Backend] Integrate telemetry metrics — Issue #940

Changes

backend/src/metrics.rs

• Added REQUEST_COUNT — a CounterVec with (method, path, status) labels that explicitly tracks the total number of HTTP requests received. This complements the existing latency histogram by providing a direct counter for request rate calculations.
• Updated init() to force-initialize the new REQUEST_COUNT metric at startup.
• Updated latency_middleware to increment REQUEST_COUNT alongside recording latency observations in the histogram.

backend/src/api.rs

• Reordered the metrics middleware layering so latency_middleware is applied before /metrics route registration. This ensures:
  • The /metrics endpoint is not tracked by the latency middleware (avoids circular metric collection on the metrics endpoint itself).
  • All API routes remain properly wrapped for request count, latency, and active-connection tracking.

Testing

• cargo check / cargo clippy — clean.
• cargo test — all 61 tests pass (34 lib, 16 api, 6 kyc_webhook, 5 middleware).
• Builds correctly with and without the metrics feature flag.

Closes #940

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[ONEONUORA]

Great job @Stanley-Owoh