No `/metrics` (Prometheus) endpoint

[Darkdruce]

Summary

The service exposes no metrics endpoint; there is no way to scrape request/settlement/webhook counters.

Location

• src/api/mod.rs (no metrics route)

Details & impact

Operators can't monitor throughput, error rates, settlement latency, or webhook success without parsing logs.

Suggested fix

Add a metrics exporter (e.g., metrics + Prometheus) with counters/histograms for the key flows.

Acceptance criteria

• /metrics exposes request, settlement, and webhook metrics.

[Hahfyeex]

Hello maintainer, I would like to work on this issue

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #133)
2. Your PR will be reviewed by the StellarGateLabs maintainers

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