protocol support for labels

pgconf.dev is coming up soon. I won't be able to make it to Montreal,
but I saw that Dave Cramer posted on twitter asking about postgres
protocol topics.

while I don't have a patch, I wanted to send an email about something:
it'd be great to have a place - perhaps like application_name - for
arbitrary labels; and we need protocol support to pass those labels
inline with the query instead of having to require a separate round
trip. it would also be good if these labels were somehow accessible in
logging formats.

observability frameworks like OpenTelemetry support tracing through all
layers of a stack, and trace_ids can even be passed into sql with
extensions like sqlcommenter. however sqlcommenter puts the trace_id
into a comment which effectively breaks all the utility of
pg_stat_statements since each query has a unique trace_id.

if protocol enhancements are on the table, I think it would be great
for this topic to get a little discussion.

-Jeremy