## Issue Addressed
NA
## Proposed Changes
In #4024 we added metrics to expose the latency measurements from a VC to each BN. Whilst playing with these new metrics on our infra I realised it would be great to have a single metric to make sure that the primary BN for each VC has a reasonable latency. With the current "metrics for all BNs" it's hard to tell which is the primary.
## Additional Info
NA
## Issue Addressed
NA
## Proposed Changes
Adds a service which periodically polls (11s into each mainnet slot) the `node/version` endpoint on each BN and roughly measures the round-trip latency. The latency is exposed as a `DEBG` log and a Prometheus metric.
The `--latency-measurement-service` has been added to the VC, with the following options:
- `--latency-measurement-service true`: enable the service (default).
- `--latency-measurement-service`: (without a value) has the same effect.
- `--latency-measurement-service false`: disable the service.
## Additional Info
Whilst looking at our staking setup, I think the BN+VC latency is contributing to late blocks. Now that we have to wait for the builders to respond it's nice to try and do everything we can to reduce that latency. Having visibility is the first step.
