feat: metric: export Mpool message count

[rjan90]

Related Issues

Partially #10547. Also nice to have when investigating things like MpoolSelect times, syncing, etc, and its potential correlations with size of the Mpool.

Proposed Changes

Recording the amount of pending messages in the mpool. The metric is updated after messages are removed from the pool (which happens on HeadChange or pruneMessage).

Additional Info

Testing: in localhost:1234/debug/metrics, you now have:

# HELP lotus_mpool_message_count Number of messages in the mpool
# TYPE lotus_mpool_message_count gauge
lotus_mpool_message_count 180

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, build, chore, ci, docs, perf, refactor, revert, style, test
  • area, e.g. api, chain, state, market, mempool, multisig, networking, paych, proving, sealing, wallet, deps
• If the PR affects users (e.g., new feature, bug fix, system requirements change), update the CHANGELOG.md and add details to the UNRELEASED section.
• New features have usage guidelines and / or documentation updates in
  • Lotus Documentation
  • Discussion Tutorials
• Tests exist for new functionality or change in behavior
• CI is green

[Stebalien]

Could you add a quick entry to the changelog (then merge)?