tracing: remove I/O type names from handshake spans

[hawkw]

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.

[olix0r]

Thanks!