Collect flashblock metrics during execution

[meyer9]

We should collect metrics on flashblock execution for flashblock-enabled validators. This will tell us how long each flashblock took to process.

https://github.com/base/node-reth/blob/6c7611a88d465f56106d15e8bd0e2a6b9e301724/crates/client/flashblocks/src/metrics.rs

For now, we can just do flashblock metrics aggregated by block number (not per-flashblock metrics), but eventually, we could also capture flashblock metrics by using fractional block numbers (1.0 is first flashblock, 1.25 is second flashblock, 1.5 is third flashblock, 1.75 is fourth flashblock).