Add byte-level metrics for P2P message exchange

[fab-10]

PR description

Built on top of #9680, please review it first.

Introduce besu_network_p2p_bytes_inbound and besu_network_p2p_bytes_outbound metrics to track
the number of bytes exchanged via P2P connections, providing better
visibility into network bandwidth usage alongside existing message counts.

Fixed Issue(s)

Thanks for sending a pull request! Have you done the following?

• Checked out our contribution guidelines?
• Considered documentation and added the doc-change-required label to this PR if updates are required.
• Considered the changelog and included an update if required.
• For database changes (e.g. KeyValueSegmentIdentifier) considered compatibility and performed forwards and backwards compatibility tests

Locally, you can run these tests to catch failures early:

• spotless: ./gradlew spotlessApply
• unit tests: ./gradlew build
• acceptance tests: ./gradlew acceptanceTest
• integration tests: ./gradlew integrationTest
• reference tests: ./gradlew ethereum:referenceTests:referenceTests
• hive tests: Engine or other RPCs modified?

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.