Monitor Stellar ledger close-time to detect network slowdowns

[Emmyt24]

Description

Track ledger close times and expose a metric/alert when they exceed normal bounds, signalling network degradation that affects deployments.

Requirements and context

Handle network failures gracefully with retries/backoff where appropriate, avoid leaking credentials, and add integration tests using mocked external services. Update configuration docs and keep coverage at target.

Suggested execution

Create a feature branch using the convention:

feat/integration-ledger-close-monitoring

Implement changes

• Poll/derive recent ledger close times.
• Expose a metric for close-time and a derived health signal.
• Alert when close-time exceeds a threshold.
• Add tests for normal and degraded close-time scenarios.

Test and commit

Run the test suite. Document the threshold in the PR.

Example commit message

feat(integration): monitor Stellar ledger close-time

Guidelines

• Follow the existing code style, linting rules, and module conventions already present in the repository.
• Keep the change focused and reviewable; avoid unrelated refactors in the same PR.
• Write clear, descriptive commit messages using the Conventional Commits format.
• Ensure all CI checks pass (build, lint, type-check, tests, coverage) before requesting review.
• Open a pull request against the default branch, link this issue (Closes #<id>), and describe what you changed and how you verified it.
• Be responsive to review feedback and keep the PR scope aligned with this issue.