kad: Add kademlia metrics

[lexnv]

Expose the following info for reporting back metrics:

• kademlia total number of records store in the memory store
• kademlia total number of bytes stored in the memory store (sum of all record's value)
• kademlia entries per bucket
  • For simplicity, this can be a Vec<(u32, size)> similar to libp2p, where u32 is the base log2 of the bucket lower bound and usize is the number of peers in that bucket

[lexnv]

We could also add here any missing "pending" peers from the the kademlia queries: https://github.com/paritytech/litep2p/blob/66aaf9dbdf60a3b3bbfda2334e2e1887af3c2315/src/protocol/libp2p/kademlia/query/find_node.rs#L119