Separate context propagation from tracing (OTEP 66)

[dyladan]

The specification now requires a global context object which stores what they are calling "cross-cutting concern values." One cross-cutting concern may be correlations, another may be tracing, and more could be added in the future.

The minimum that needs to be done:

• Scope managers need to be modified to propagate this new Context object rather than directly propagating spans as the currently do
• Propagators need to be updated to accept these context objects rather than directly accepting span context as the currently do