Node-side metrics for asynchronous backing

[slumber]

There're several places with a potential of being a hotspot. We should introduce metrics for them to aid debugging during the testing phase.
Namely:

polkadot/node/core/prospective-parachains/src/lib.rs

Lines 245 to 247 in 0f3ef99

	if !update.deactivated.is_empty() {
	// This has potential to be a hotspot.
	prune_view_candidate_storage(view);

polkadot/node/network/collator-protocol/src/validator_side/mod.rs

Lines 1325 to 1332 in 0f3ef99

	// Re-trigger previously failed requests again.
	//
	// This makes sense for several reasons, one simple example: if a hypothetical depth
	// for an advertisement initially exceeded the limit and the candidate was included
	// in a new leaf.
	let maybe_unblocked = std::mem::take(&mut state.blocked_advertisements);
	// Could be optimized to only sanity check new leaves.
	request_unblocked_collations(sender, state, maybe_unblocked).await;

And maybe some more (@rphmeier ?)

[mrcnski]

Closed by #6549.