## Motivation

Currently, the `tracing` spans for the client and server handshakes
contain the name of the I/O type. In some cases, where nested I/O types
are in use, these names can be quite long; for example, in Linkerd, we
see log lines like this:

```
2022-03-07T23:38:15.322506670Z [ 10533.916262s] DEBUG ThreadId(01) inbound:accept{client.addr=192.168.1.9:1227}:server{port=4143}:direct:gateway{dst=server.echo.svc.cluster.local:8080}:server_handshake{io=hyper::common::io::rewind::Rewind<linkerd_io::either::EitherIo<linkerd_io::sensor::SensorIo<linkerd_io::prefixed::PrefixedIo<linkerd_io::either::EitherIo<tokio_rustls::server::TlsStream<linkerd_io::either::EitherIo<linkerd_io::scoped::ScopedIo<tokio::net::tcp::stream::TcpStream>, linkerd_io::prefixed::PrefixedIo<linkerd_io::scoped::ScopedIo<tokio::net::tcp::stream::TcpStream>>>>, linkerd_io::either::EitherIo<linkerd_io::scoped::ScopedIo<tokio::net::tcp::stream::TcpStream>, linkerd_io::prefixed::PrefixedIo<linkerd_io::scoped::ScopedIo<tokio::net::tcp::stream::TcpStream>>>>>, linkerd_transport_metrics::sensor::Sensor>, linkerd_io::sensor::SensorIo<linkerd_io::either::EitherIo<tokio_rustls::server::TlsStream<linkerd_io::either::EitherIo<linkerd_io::scoped::ScopedIo<tokio::net::tcp::stream::TcpStream>, linkerd_io::prefixed::PrefixedIo<linkerd_io::scoped::ScopedIo<tokio::net::tcp::stream::TcpStream>>>>, linkerd_io::either::EitherIo<linkerd_io::scoped::ScopedIo<tokio::net::tcp::stream::TcpStream>, linkerd_io::prefixed::PrefixedIo<linkerd_io::scoped::ScopedIo<tokio::net::tcp::stream::TcpStream>>>>, linkerd_transport_metrics::sensor::Sensor>>>}:FramedWrite::buffer{frame=Settings { flags: (0x0), initial_window_size: 65535, max_frame_size: 16384 }}: h2::codec::framed_write: send frame=Settings { flags: (0x0), initial_window_size: 65535, max_frame_size: 16384 }
```

which is kinda not great.

## Solution

This branch removes the IO type's type name from the spans for the
server and client handshakes. In practice, these are not particularly
useful, because a given server or client instance is parameterized over
the IO types and will only serve connections of that type.