[#498] Set up Prometheus metrics endpoint (`GET /metrics`) and Grafana dashboard for API observability

[AdaBebe0]

Backlog reference: #498 · infra perf

Summary

Set up Prometheus metrics endpoint (GET /metrics) and Grafana dashboard for API observability

Context

The final go/no-go checklist before real money moves on Stellar mainnet: observability, an external security audit, and the mainnet cutover itself.

The path to a public, mainnet launch: KYC/compliance groundwork, price/localization/mobile polish, performance hardening, documentation, and the final production go-live checklist.

Acceptance Criteria

• Set up Prometheus metrics endpoint (GET /metrics) and Grafana dashboard for API observability — implemented as described above.
• The change has been exercised locally the way a contributor actually would (docker-compose up, the relevant make target, etc.).
• If it changes the contributor workflow, CONTRIBUTING.md or README.md is updated to match.
• A before/after measurement (timing, query plan, load-test result) is included in the PR description.

Definition of Done

• Implemented on a branch and opened as a PR referencing this issue (Closes #498 won't auto-link since this is a repo-internal reference — mention the issue number GitHub assigned instead).
• CI is green (lint, typecheck, tests, build).
• Reviewed against CONTRIBUTING.md conventions (Conventional Commits, code style).
• CHANGELOG.md updated under [Unreleased] if the change is user- or API-visible.
• Merged to main.

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Generated from the CoopLumen backlog (issue.md). See CONTRIBUTING.md for the contribution workflow.