feat(discv5): peers per kbucket metric

[emhane]

Ref #3665

• Adds metric for keeping track of how many peers are in each discv5 kbucket

todo: add intermediary struct for not holding read lock on kbuckets at same time as writing to gauges
optional: keep track of lowest kbucket index with a peer to skip reading kbucket peer count for any buckets bellow it (lower indices are likely to be empty buckets)