B3 propagator package does not confirm to spec.

[owais]

According to the spec, OTEL_PROPAGATORS env var should support two types of B3 propagators, b3 and b3multi.

b3 is supposed to inject a single header with - separate values. https://github.com/openzipkin/b3-propagation#single-header
b3multi is supposed to inject one header per value. https://github.com/openzipkin/b3-propagation#multiple-headers

Currently, we have a single implement at opentelemetry.propgators.b3.B3Format which actually is the multi-header implementation.

This implementation is exposed as an propagator entry-point with the name b3 instead of b3multi.

This means setting OTEL_PROPAGATORS=b3 will actually use a multi-header B3 propagator instead of single header while as setting OTEL_PROPAGATORS=b3multi will raise an exception.

IMO, this is a bug and should be fixed as follows:

1. Add two new propagators called B3SingleFormat and B3MultiFormat.
2. Deprecate the existing B3Format propagator without changing behavior. Deprecation notice should recommend using B3MultiFormat.
3. Change b3 entrypoint to point to B3SingleFormat implementation (breaking behavioral change).
4. Add a new b3multi entrypoint to point to B3MultiFormat implementation.