Match OpenTelemetry core propagator functionality with baggage/traceparent

[jkwatson]

Is your feature request related to a problem? Please describe.
The core OpenTelemetry W3C propagators operate completely independently of each other. That is, the W3CBaggagePropagator will extract baggage off the wire and put it into the Context, whether or not there is a valid Span in the incoming headers that could be extracted by the W3CTraceContextPropagator.

Currently the sleuth otel implementation will only provide Baggage in the context if there is also a valid Span in the traceparent header.

Describe the solution you'd like
It would be good to match the core OpenTelemetry functionality and be able to extract Baggage independently of whether or not there is a Span in the incoming payload.

[jkwatson]

Feel free to assign this to me, if y'all agree we should address it.