fatxpool: add per-transaction event-timings metrics

[michalkucharczyk]

Currently there is no way to get metrics/timings related to the transaction lifecycle events.

It could be beneficial to have those numbers exposed in order to build transaction-pool reliability dashboard and extend existing grafana board. This in turn could be used to detect service degradation or unusual patterns.

Metrics could work for both RPC nodes (for txs sent over RPC) and validators (for txs gossiped over p2p).
RPC stats would reflect the end user experience, validators stats would reflect the transaction processing pace.

I would measure and expose cumulative stats for all transaction - per event in the form of prometheus buckets.