fix: Unparent connection span

[Jesse-Bakker]

The Connection struct contains a tracing::Span tracking its
lifetime. This can be useful. The problem is that this span has as its
parent the span within which the connection is created, meaning that it
keeps that span alive (not closed) while it is alive. In practice that
means, for example, instrumented functions that create connections and
store those connections for reuse will not have their spans closed and
exported when they end.

This shows up in tonic, where the first RPC call to any server will have its containing span alive for the remainder of the RPC client's lifetime.

This commit sets the Connection's span's parent to None and gives it
a "follows-from" relation to the span in which it was created instead.
This shows the causal relationship between them, without tying the
creating span's lifetime to the connection's lifetime.

[seanmonstar]

@cratelyn hey, I know linkerd2-proxy was originally interested in tracing with h2, how would this affect your workflow?

[cratelyn]

thanks for flagging me @seanmonstar! this looks like a great change to me, and i agree with the rationale outlined in the description here. this would be an ergonomic improvement for span lifetimes, and i support this change!

[seanmonstar]

Right on, I'm good with releasing this and if causes issues, someone can tell us and we can evaluate more closely. :)