Propagate span values to children spans

[mijicd]

There should be possible to automatically propagate some values (e.g. correlation ids) from parent to its children. I propose to make this the default behavior of put, and to add putLocal which should be used to add span-local values.

[tpolecat]

I think there are three scopes that matter:

• span - local to the span, for operation-specific attributes (method arguments)
• local subtree - propagated to the subtree but not across remote calls, for machine-specific attributes (local uri, hostname)
• global subtree - propagated to the subtree, including remote calls (trace id, initial uri, user)

I have a WIP that requires you to specify the scope with put. I think there probably shouldn't be a default.

[mijicd]

These looks even better. I mentioned default behavior because my proposal was to have 2 separate methods (put and putLocal) where put would the propagation.