feat(metrics): add Prometheus latency histogram for /api/markets

[MJ-RWA]

Summary

Adds Prometheus request latency instrumentation for the /api/markets endpoint using a prom-client Histogram with explicit buckets and route labels. This improves observability by enabling latency tracking and performance analysis while preserving existing endpoint behavior.

Closes #28

Changes Made

• Added a Prometheus Histogram for /api/markets request latency.

• Registered the histogram in the metrics registry using the project's existing metrics infrastructure.

• Configured explicit latency buckets for accurate request duration measurement.

• Added route labels (and other standard labels where applicable) to support filtering and aggregation.

• Instrumented the /api/markets endpoint to record request latency for both successful and failed requests.

• Prevented duplicate metric registration to ensure compatibility with repeated initialization and test environments.

• Preserved standardized error handling and existing API behavior.

• Integrated with the project's structured logging and correlation ID conventions where applicable.

• Added focused tests covering:

  • Histogram registration
  • Request latency observation
  • Route label assignment
  • Explicit bucket configuration
  • Successful and failed request scenarios
  • Duplicate registration safeguards

• Updated relevant documentation to describe the new latency metric.

Testing

• Added unit and integration tests for the latency histogram.
• Verified metrics are recorded correctly for /api/markets.
• Confirmed no changes to existing endpoint responses or error handling.
• Ran the project's linting and test suites successfully.

Notes

This change is fully backward compatible and introduces no API-breaking changes. It adds internal Prometheus latency metrics to improve monitoring and operational visibility for the /api/markets endpoint.

Closes: #602

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[greatest0fallt1me]

Clean PR — merged 🙌