`StreamMuxerBox` is never shared across threads but owned by a single
connection. Restricting it to be `Sync` unnecessarily limits the design
space around the `StreamMuxer` trait and its implementations.
Instead of having a mix of `poll_event`, `poll_outbound` and `poll_close`, we
flatten the entire interface of `StreamMuxer` into 4 individual functions:
- `poll_inbound`
- `poll_outbound`
- `poll_address_change`
- `poll_close`
This design is closer to the design of other async traits like `AsyncRead` and
`AsyncWrite`. It also allows us to delete the `StreamMuxerEvent`.
* 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.
Remove the concept of individual `Transport::Listener` streams from `Transport`.
Instead the `Transport` is polled directly via `Transport::poll`. The
`Transport` is now responsible for driving its listeners.
**Summary** of the plot of the `discover_peer_after_disconnect` test:
1. `swarm2` connects to `swarm1`.
2. `swarm2` requests an identify response from `swarm1`.
3. `swarm1` sends the response to `swarm2`.
4. `swarm2` disconnects from `swarm1`.
5. `swarm2` tries to disconnect.
**Problem**
`libp2p-identify` sets `KeepAlive::No` when it identified the remote. Thus `swarm1` might
identify` `swarm2` before `swarm2` identified `swarm1`. `swarm1` then sets `KeepAlive::No` and thus closes the
connection to `swarm2` before `swarm2` identified `swarm1`. In such case the unit test
`discover_peer_after_disconnect hangs indefinitely.
**Solution**
Add an initial delay to `swarm1` requesting an identification from `swarm2`, thus ensuring `swarm2`
is always able to identify `swarm1`.
See e04e95a for the rational.
With tokio `v1.19.0` released, `TcStream` exposes `take_error`.
This commit applies the same fix from e04e95a to the tokio TCP provider.
* core/muxing: Remove `Into<io::Error>` bound from `StreamMuxer::Error`
This allows us to preserve the type information of a muxer's concrete
error as long as possible. For `StreamMuxerBox`, we leverage `io::Error`'s
capability of wrapping any error that implements `Into<Box<dyn Error>>`.
* Use `?` in `Connection::poll`
* Use `?` in `muxing::boxed::Wrap`
* Use `futures::ready!` in `muxing::boxed::Wrap`
* Fill PR number into changelog
* Put `Error + Send + Sync` bounds directly on `StreamMuxer::Error`
* Move `Send + Sync` bounds to higher layers
* Use `map_inbound_stream` helper
* Update changelog to match new implementation
