BUG/MINOR: mux-h2/traces: bring back the lost "sent H2 REQ/RES" traces

BUG/MINOR: mux-h2/traces: bring back the lost "sent H2 REQ/RES" traces

In 2.4, commit d1ac2b90c ("MAJOR: htx: Remove the EOM block type and
use HTX_FL_EOM instead") changed the HTX processing to destroy the
blocks as they are processed. So the traces that were emitted at the
end of the send headers functions didn't have anything to show.

Let's move these traces earlier in the function, right before the HTX
processing, so that everything is still in place.

This should be backported to 2.4.