Ensure to prune to target peers

[twoeths]

Describe the bug

In rare case where all peers subscribe to at least 1 subnet, but they are not grouped to 1 subnet, and all performs well, lodestar doesn't prune peers to target peers. When that happens, we see peers stay >= 55 peers like below:

this does not allow new connections (implemented by libp2p). in worse case scenario, we don't have enough subnet peers (from same node like the above)

and it causes a lot of missed attestations (since this node has 1k validators on goerli and it's supposed to subscribe to all subnets)

Expected behavior

Make sure we always prune to target peers (50) to allow more better peers to come, like below:

[twoeths]

Adding no long lived subnet peers metric which is used to find this issue

Requested peer to disconnect metric is like

[twoeths]

when this error happens, it leads to InsufficientPeers issue at validator side

grep -e "PublishError.InsufficientPeers" -rn validator-2023-02-23.log | wc -l
20070