chore: Observability improvements to node logs and metrics | NPG-7984

[saibatizoku]

Description

Update a couple of log messages to clarify fragments rejected from the mempool.
Convert lastBlockTx prometheus metric from being a gauge into counter.
Add docstrings to clarify the meaning of prometheus metrics related to transactions.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[FelipeRosa]

LGTM