Upgrade to use released version of Opentelemetry and also pushes the span context to gracefully handle multiple routers

[paulanthonywilson]

Based on this unmerged PR to the base, but seemingly abandoned, original repo opentelemetry-beam#15 . In addition to what is advertised in the PR it updates to depend on the actually released version of Opentelemetry which reduces the dependency hell when trying to upgrade a services dependencies.

Small changes to the PR are:

• A rebase to the current master
• Loosen dependency requirements to make it easier to include with other deps.

From the original PR:

In the previous version, I did not anticipate the possibility that one router
might delegate to another for code-organization reasons, and so we would enter
the telemetry handler code twice while trying to service a request and lose the
parent context we attempted to save on the first time around. We witnessed this
as traces in our aggregator/visualizer that were missing root spans.

This change brings the code closer to what opentelemetry_telemetry does without
adding that dependency; namely, it maintains the parent context as a list/stack
and pushes onto it when a new span is created, popping it when a span completes.
Also, if a context already exists, we don't attempt to extract propagation from
the request headers. This change will also make the library more compatible with
webserver instrumentation like opentelemetry_cowboy, which does the header
extraction and context creation.

[pedro-gutierrez]

Looks good to me!

[jamcito]

LGTM!