misc/metrics: Add support for libp2p-gossipsub

[mxinden]

A draft adding minimal support for libp2p-gossipsub to libp2p-metrics.

I think a first version should at least instrument all variants of GossipsubEvent.

rust-libp2p/protocols/gossipsub/src/behaviour.rs

Lines 115 to 144 in 937b59d

	/// Event that can be emitted by the gossipsub behaviour.
	#[derive(Debug)]
	pub enum GossipsubEvent {
	/// A message has been received.
	Message {
	/// The peer that forwarded us this message.
	propagation_source: PeerId,
	/// The [`MessageId`] of the message. This should be referenced by the application when
	/// validating a message (if required).
	message_id: MessageId,
	/// The decompressed message itself.
	message: GossipsubMessage,
	},
	/// A remote subscribed to a topic.
	Subscribed {
	/// Remote that has subscribed.
	peer_id: PeerId,
	/// The topic it has subscribed to.
	topic: TopicHash,
	},
	/// A remote unsubscribed from a topic.
	Unsubscribed {
	/// Remote that has unsubscribed.
	peer_id: PeerId,
	/// The topic it has subscribed from.
	topic: TopicHash,
	},
	/// A peer that does not support gossipsub has connected.
	GossipsubNotSupported { peer_id: PeerId },
	}

In the long run we should extend GossipsubEvent to e.g. expose details about the mesh.

Example on how to use libp2p-metrics can be found here: https://github.com/libp2p/rust-libp2p/blob/master/misc/metrics/examples/metrics.rs

A binary using libp2p-metrics is e.g. https://github.com/mxinden/rust-libp2p-server/

[mxinden]

Merged via #2316.