Previously, we would only track the metrics like the number of open connections. With this patch, we extend these metrics with a `protocols` label that contains a "protocol stack". A protocol stack is a multi-address with all variable parts removed. For example, `/ip4/127.0.0.1/tcp/1234` turns into `/ip4/tcp`.
Resolves https://github.com/libp2p/rust-libp2p/issues/2758.
* misc/metrics: Explicitly delegate event recording to each recorder
This allows delegating a single event to multiple `Recorder`s. That enables e.g. the
`identify::Metrics` `Recorder` to act both on `IdentifyEvent` and `SwarmEvent`. The latter enables
it to garbage collect per peer data on disconnects.
* protocols/dcutr: Expose PROTOCOL_NAME
* protocols/identify: Expose PROTOCOL_NAME and PUSH_PROTOCOL_NAME
* protocols/ping: Expose PROTOCOL_NAME
* protocols/relay: Expose HOP_PROTOCOL_NAME and STOP_PROTOCOL_NAME
* misc/metrics: Track # connected nodes supporting specific protocol
An example metric exposed with this patch:
```
libp2p_identify_protocols{protocol="/ipfs/ping/1.0.0"} 10
```
This implies that 10 of the currently connected nodes support the ping protocol.
