This PR extracts opentelemetry utility functions from my private project
and adds them to this project without calling them. It resolves rabbitmq#43

I'd like a broader discussion about whether these should be
automatically called by the library where possible, or if they should
simply be provided to clients to use if they so wish.

I did my best to follow OpenTelemetry semantic conventions as described
here
https://opentelemetry.io/docs/specs/semconv/messaging/messaging-spans/,
but they are at times ambiguous for rabbitmq-- e.g. is the destination
for a message the Queue or the Consumer Tag the message was delivered to.

Given the channel based approaches of this library, it is impossible for
the library to know the full execution of a consumer. Unless
autoack=false, we cannot actually know when to end the span associated
with a delivery, so at least in the consumer case, it's probably best to
allow the client to manage spans for themselves.

We *can* manage spans on the producer side, and at the very least
extract span identifiers to include on published headers automatically,
and provide utilities for pulling them back out again.

My intention with putting this PR up is to move the conversation
forward. Because the PR *only* provides private methods (if I left
members public please call them out), it can be safely merged while
these questions are worked out.

…an/amqp091-go into feat/opentelemetry

…an/amqp091-go into feat/opentelemetry