Baggage span processor - key predicate

[MikeGoldsmith]

Enhancement idea to the Baggage Span Processor to allow configuration of a user-provided key predicate function used to determine whether a baggage entry should be added to newly started spans.

          I think this behavior can be considered not safe for some organizations.

Wondering if you considered as an alternative adding a config option that can filter out attributes for collection. Perhaps some function that receives an attribute name and return if it should be recorder or not?

export interface BaggageSpanProcessorConfig {
  attributeRecording(baggaeAttributeName: string): boolean;
}

and then users can opt-in to collect only attributes they know are safe, or specifically feed a function that returns true to record everything?

This can shift the responsibility of what is being collected to the user, allowing safe flexibility in collection without introducing breaking changes in the future.

Originally posted by @blumamir in #2114 (comment)

[blumamir]

Adding here the comment from the PR, that changing the default behaviour for baggage key collection in the future to a "safe one" will probably be a breaking change for this component.