Implemented logic for Opentracing baggage propagation

[jaronoff97]

What

This PR implements the logic for extracting baggage in the opentracing propagator.

Why

Users of the opentracing baggage propagator want to extract baggage so that they can effectively migrate from opentracing to opentelemetry.

Other Implementations:

Java
Python
Javascript

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Jacob Aronoff (3db3dc3, 9f13bf5)

[jaronoff97]

Pending CLA authorization from Lightstep right now.

[codecov]

Codecov Report

Merging #1880 (170e137) into main (d1b7211) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #1880     +/-   ##
=======================================
+ Coverage   70.5%   70.6%   +0.1%     
=======================================
  Files        129     129             
  Lines       5564    5587     +23     
=======================================
+ Hits        3925    3948     +23     
  Misses      1500    1500             
  Partials     139     139             

Impacted Files	Coverage Δ
propagators/ot/ot_propagator.go	94.5% <100.0%> (+1.8%)	⬆️

[carlosalberto]

Looks great, thanks!

[jaronoff97]

@carlosalberto one result of this behavior is that if baggage from the opentracing side is invalid, we don't create a context from remote span. Should we instead change this block to be something like:

         bags, err := extractBags(carrier)
	if err != nil {
		return trace.ContextWithRemoteSpanContext(ctx, sc)
	}
	ctx = baggage.ContextWithBaggage(ctx, bags)
	return trace.ContextWithRemoteSpanContext(ctx, sc)

[carlosalberto]

@jaronoff97 Oh, good call - indeed, invalid Baggage shouldn't prevent a valid SpanContext from being returned.

[hanyuancheung]

lgtm