Implement OpenTracing bridge

[alban]

This tracer implements the OpenTracing API using OpenTelemetry internally.

This includes unit tests too.

---

TODO:

• documentation
• start time & end time
• OTel's links / OT's references
• Baggage: what to do here?
• Logs: what to do here?
• Context propagation to other processes: which http headers to use?

[carlosalberto]

Hey,

Thanks for the PR, but this is not the approach we are looking for - first of all, I don't see why we need to have a dependency to basictracer (doing so also would mean keeping actually two tracers: an OpenTracing and an OpenTelemetry one).

Second, we only want to expose publicly a way to retrieve an OpenTracing out of an OpenTelemetry Tracer, nothing else:

import opentracingshim

tracer = opentracingshim.create_tracer(io.opentelemetry.tracer())
with tracer.start_active_span("one") as scope:
  scope.span.set_tag("my", "tag")

Which means we cannot any actual implementation of ScopeManager and other members exposed by the OpenTracing API, but simply provide a set of wrapper classes around the OTel components.

See how the WIP looks for Java for reference (mostly missing OT Baggage support): https://github.com/open-telemetry/opentelemetry-java/tree/master/opentracing_shim/src/main/java/io/opentelemetry/opentracingshim (you don't have to port this, as there are enough differences).

[johananl]

Updating here for visibility - I am continuing @alban's work on this following the feedback from @carlosalberto. I shall open a new PR with an initial implementation soon (I can also update this one if preferred).

[johananl]

Superseded by #211. This PR can be closed.