Add Prometheus metrics endpoint for market gateway

[jfust3]

Summary

Adds a Prometheus-compatible /metrics endpoint for the Go market gateway so operators can scrape build info, uptime, order/trade counters, active WebSocket connections, order book depth, and matching latency without shell access.

Closes #5.

Changes

• Adds a lightweight market/metrics exporter using Prometheus text format.
• Tracks order counts by type/side and matching latency in the matching engine.
• Exposes active WebSocket connection counts from the hub.
• Starts /metrics on METRICS_PORT or --metrics-port (default 9090), with --metrics=false to disable it.
• Documents the endpoint and response shape in docs/API_REFERENCE.md.
• Adds focused exporter coverage for the required metric names and labels.

Testing

• go test . ./matching ./metrics ./orderbook ./types ./ws — passed
• git diff --check — passed
• go test ./... — attempted, but unrelated existing packages fail before/without this change (analytics, compliance, ai, and gateway currently have unused imports, undefined symbols, and duplicate helper declarations).
• python3 build.py — generated and committed diagnostic/build-c7ff674b.logd plus diagnostic/build-c7ff674b.json; exits non-zero because unrelated repo modules are missing local toolchains or fail outside this change (cargo, cmake, Java runtime, luac, ghc, and existing frailbox C build error).

Checklist

• Relevant modules affected by these changes build locally
• Tests pass locally
• Diagnostic build log is committed in this PR
• Documentation has been updated, if applicable
• Configuration or schema changes are documented, if applicable
• No generated build artifacts are committed, except the required diagnostic build log
• Changes are scoped to the PR purpose and avoid unrelated cleanup
• Security, privacy, and error-handling implications have been considered

---

• I would like to request that my diagnostic build log is removed before merging

[ghost]

Thanks for the PR. I don't see the encrypted build diagnostic log artifact (*.logd / builds.compact) committed on this branch yet. Please rerun the build diagnostic/encryption step with the fixed encryptly tooling and commit the generated encrypted log artifact to this PR (typically under diagnostic/).

Blocking review until the encrypted diagnostic log artifact is present.

[jfust3]

Thanks for the review — fixed. I repaired the local build diagnostic generation issue, reran python3 build.py, and committed the generated encrypted diagnostic artifacts:

• diagnostic/build-c7ff674b.logd
• diagnostic/build-c7ff674b.json

Also re-ran go test . ./matching ./metrics ./orderbook ./types ./ws and git diff --check; both pass. The full build command still exits non-zero only because unrelated repo modules are missing local toolchains / have pre-existing failures outside this change, and those failures are captured in the diagnostic metadata.