PrometheusSimple backend robustness improvements

[mgmeier]

Description

This PR addresses issue #6389. It aims to mitigate unresponsiveness or even crashes of the Node's internal PrometheusSimple metrics backend that may be caused by factors in the host environment.

It contains the following changes to the backend, improving robustness:

• Make the backend auto-retry/-restart when startup fails or its thread receives an exception
• Trace start and stop events of the backend to provide basic observability
• Be more eager in reaping dangling socket connections
• Fine-tune the backend's DoS protection settings somewhat

It has no impact on Node configuration or CLI arguments.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • roundtrip tests
  • integration tests
    See Runnings tests for more details
• Any changes are noted in the CHANGELOG.md for affected package
• The version bounds in .cabal files are updated
• CI passes. See note on CI. The following CI checks are required:
  • Code is linted with hlint. See .github/workflows/check-hlint.yml to get the hlint version
  • Code is formatted with stylish-haskell. See .github/workflows/stylish-haskell.yml to get the stylish-haskell version
  • Code builds on Linux, MacOS and Windows for ghc-9.6 and ghc-9.12
• Self-reviewed the diff

[Russoul]

LGTM