feat: Add custom metric to track whether a message processed.

[dianakhuang]

New custom metric kafka_received_message added to span contexts.

Merge checklist: Check off if complete or not applicable:

• [x] Version bumped
• [x] Changelog record added
• [x] Documentation updated (not only docstrings)
• [x] Commits are squashed
• [x] Noted any: Concerns, dependencies, deadlines, tickets, testing instructions