Functions for modifying a Composite Propagator

[alanwest]

The ability to create a composite propagator is specified. In .NET, instrumentation defaults to use a composite propagator that supports propagating W3C trace context and baggage.

Currently, if a user wants to enable an additional propagator (e.g., Zipkin's B3 format), but also maintain the W3C trace context and baggage propagators, they would need to create a new composite propagator and make sure they add all three.

I'd like to suggest adding a means to modify an existing composite propagator. Modifications like adding to, removing from, and perhaps clearing the list of propagators. It may also be nice to have "contains" functionality to check if a composite propagator contains a certain type of propagator.