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)

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• Considered documentation and added the doc-change-required label to this PR if updates are required.
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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
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[macfarla]

@fab-10 any performance impact from this?

[fab-10]

@fab-10 any performance impact from this?

I have seen none