Baggage span processor - key predicate

[MikeGoldsmith]

This issue is to track adding a method of selecting what baggage key entries should be copied.

Feedback in the JS contrib PR was to allow a user-provided predicate function. This puts the responsibility on the user to ensure sensitive baggage keys are not copied while also not prescribing how that is determined.

• Baggage span processor - key predicate opentelemetry-js-contrib#2166

We had a similar feedback in the .NET contrib project but thought it was more complicated than just using a set of prefixes so created an issue to continue the discussion. The plain processor that copies all baggage entries (like using * in your example) is likely to be accepted first.

• Baggage activity processor - key predicate opentelemetry-dotnet-contrib#1695