Networking metrics to add to Prometheus

[tomaka]

To add:

• Number of records in the Kademlia storage. (xcm-executor: validate destinations for ReserveWithdraw and Teleport transfers #5660)
• Number of active Kademlia iterative queries.
• Number of active Kademlia RPC queries for which we're waiting for an answer.
• Number of open connections that are being kept alive but unused at this time.
• Number of open connections that are being used for Kademlia queries.
• Number of open connections that are being used for light client requests.
• Number of open connections that are being used for syncing (new protocol).
• Number of open connections that are being used for notifications (for each notification protocol).
• Sizes of notifications ('remainder' instead of reminder && explicit instruction to clone #5535)
• Sizes of light client requests and responses.
• Sizes of block requests (new protocol).
• On the requesting side, delay between light client request and response.
• On the requesting side, delay between block request and response.

[tomaka]

I re-purposed this issued as a more generic "networking metrics to add" rather than Kademlia-specific.

[tomaka]

cc libp2p/rust-libp2p#1701