Support Bagagge-only propagation in the OpenTracing Shim.

[carlosalberto]

Resolves an item from #2147

Although many OpenTracing tracers do not support baggage propagation amidst missing/invalid SpanContext, this is something (at least) Jaeger clients expect.

Overall, the changes to properly support this are:

1. Extract and inject MUST handle the case of valid Baggage, even amidst missing/invalid SpanContext, in order to support the vanilla OT propagation case:

// extractedSpanContext is invalid, but it contains non-empty Baggage
SpanContext extractedSpanContext = tracer.extract(carrier, Format.TEXT_MAP, textMapAdapter);
Span span1 = tracer.buildSpan("child").asChildOf(extractedSpanContext).start();

2. activeSpan() MUST include Baggage handling, in order to support OTel/OT explicit propagation:

// OTel extracts and activates Baggage, SpanContext is missing.
try (io.opentelemetry.context.Scope scope = extractedContext.current()) {
  ...
  // OT **explicitly** tries to fetch any current Span as the parent.
  // span1's Baggage MUST consume the Baggage in the extracted Context.
  Span span1 = tracer.buildSpan("child").asChildOf(tracer.activeSpan()).start();
}

3. startSpan() MUST consume any active Baggage, in order to support OTel/OT implicit propagation:

// OTel extracts and activates Baggage, SpanContext is missing.
try (io.opentelemetry.context.Scope scope = extractedContext.current()) {
  ...
  // OT **implicitly** tries to fetch any current Span as the parent.
  // span1's Baggage MUST consume the Baggage in the extracted Context.
  Span span1 = tracer.buildSpan("child").start();
}

See the prototype for Java: https://github.com/carlosalberto/opentelemetry-java/compare/otshim-spanshim-lock...carlosalberto:otshim-baggage-only-support?expand=1

[jmacd]

Thanks for the detailed PR description 😀