The fetcher should not fetch transactions that are already on chain.
Until now we were only checking in the txpool, but that does not have
the old transaction. This was leading to extra fetches of transactions
that were announced by a peer but are already on chain.
Here we extend the check to the chain as well.
This PR introduces two new metrics to monitor slow peers
- One tracks the number of slow peers.
- The other measures the time it takes for those peers to become
"unfrozen"
These metrics help with monitoring and evaluating the need for future
optimization of the transaction fetcher and peer management, for example i
n peer scoring and prioritization.
Additionally, this PR moves the fetcher metrics into a separate file,
`eth/fetcher/metrics.go`.
