Add flashblocks metrics for current flashblock

[mattsse]

trafficstars

Describe the feature

in

https://github.com/paradigmxyz/reth/blob/7a599dc13018acc7f7d206581bf4cd4eb6368091/crates/optimism/flashblocks/src/service.rs#L385-L392

we want some additional metrics type that tracks:

1. current block height
2. current index
3. histogram with execution duration by index

Additional context

No response

[reject-i]

I can try

[leniram159]

🙋