MINOR: mux-h2/traces: add missing flags and proxy ID in traces

MINOR: mux-h2/traces: add missing flags and proxy ID in traces

H2 traces are unusable to detect bugs most of the time because they miss
the h2c and h2s flags, as well as the proxy, which makes it very hard to
figure if the info comes from the client or the server as soon as two
layers are stacked. This commit adds these precious information as well
as the h2s's rx and tx windows.

This could be backported to a few recent branches, but the rx window
calculation will have to be replaced with the static value there.