[OT Shim] Fully support Baggage-only propagation.

[carlosalberto]

Baggage without a valid Span should still be properly propagated in inter-process and in-process operations.

This is part of the OT compatibility section - essentially, the changes are:

1. ActiveSpan():

If the current OpenTelemetry Span's SpanContext is invalid but the current Baggage is not empty, this operation MUST return a new Span Shim containing a no-op OpenTelemetry Span and the non-empty Baggage.

2. Tracer.Extract()

If the extracted SpanContext is invalid AND the extracted Baggage is empty, this operation MUST return a null value, and otherwise it MUST return a SpanContext Shim instance with the extracted values.

cc @zeitlinger

[codecov]

Codecov Report

Base: 90.77% // Head: 90.77% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (58a3ad8) compared to base (6c11793).
Patch coverage: 90.90% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #4739   +/-   ##
=========================================
  Coverage     90.77%   90.77%           
- Complexity     4815     4816    +1     
=========================================
  Files           551      551           
  Lines         14356    14358    +2     
  Branches       1395     1395           
=========================================
+ Hits          13031    13033    +2     
  Misses          909      909           
  Partials        416      416           

Impacted Files	Coverage Δ
...opentelemetry/opentracingshim/SpanBuilderShim.java	77.89% <ø> (ø)
...pentelemetry/opentracingshim/ScopeManagerShim.java	94.44% <85.71%> (ø)
.../io/opentelemetry/opentracingshim/Propagation.java	87.50% <100.00%> (+0.40%)	⬆️
...ava/io/opentelemetry/opentracingshim/SpanShim.java	87.61% <100.00%> (+0.11%)	⬆️
...ava/io/opentelemetry/sdk/internal/RateLimiter.java	94.11% <0.00%> (-5.89%)	⬇️
...telemetry/sdk/common/InstrumentationScopeInfo.java	100.00% <0.00%> (ø)
...elemetry/sdk/testing/assertj/AttributesAssert.java	98.86% <0.00%> (ø)
...try/exporter/zipkin/ZipkinSpanExporterBuilder.java	100.00% <0.00%> (ø)
.../opentelemetry/exporter/prometheus/Serializer.java	86.44% <0.00%> (+0.42%)	⬆️

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[jack-berg]

Just one minor comment

[carlosalberto]

Anything else to merge this PR?

[jack-berg]

Looks good - Sorry about the delay @carlosalberto!