Remove possibly redundant information from span info

[oli-obk]

Right now a span entry prints something like

path::to::span::name{field=value, field2=value}

or in the case of no fields and just a message

path::to:span::name{message="foo"}

@RalfJung brought this up in rust-lang/rust#75143 (comment)

Basically we could leave the { and } off, as there's no confusion about what belongs to the span and what not like in the regular tracing printer. Additionally the message="foo" could just be "foo" or even foo.