litep2p: Increment random walk metrics (#4929)

This PR exposes the `RandomKademliaStarted` event from the litep2p
network backend, and then increments the appropriate metrics.

This is part of: https://github.com/paritytech/polkadot-sdk/issues/4681.
However, it is more of an effort to debug low peer count 

### Testing Done
- Started a node and fetched queries:
`substrate_sub_libp2p_kademlia_random_queries_total` produces results
for litep2p backend

cc @paritytech/networking

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Signed-off-by: Alexandru Vasile <[email protected]>