les: fix UDP connection query (#22451)

This PR fixes multiple issues with the UDP connection pre-negotiation feature:

- the enable condition was wrong (it checked the existence of the DiscV5 struct where it wasn't initialized yet, disabling the feature even if discv5 was enabled)
- the server pool queried already connected nodes when the discovery iterators returned them again
- servers responded positively before they were synced and really willing to accept connections

Metrics are also added on the server side that count the positive and negative replies to served connection queries.