Public span fields cannot be safely accessed

[elijahandrews]

We have a bunch of public fields on Span that cannot be safely accessed in multi-threaded code. These fields include Error, Name, Meta, and Metrics. We can't even ask the user to lock the span themselves before accessing these fields, as the mutex that protects them is private.

This is particular concerning for Meta and Metrics, as concurrent hash accesses can cause panics in newer versions of Go.

We should make these fields private and write public, thread-safe getters and setters to access this data.

[palazzem]

@elijahandrews I'm going to change our public API since our Tracer should not expose some fields now that we're moving to OpenTracing. This issue should be addressed as a consequence.

[palazzem]

@elijahandrews the new OpenTracing API (#126) returns a Span interface that sets the underlying fields using the span.SetTag(k,v) method. In that case, a mutex will be used both for updating the Meta and other fields (Service, Resource, Type).

Since we're moving towards a OpenTracing-only API, we can consider this issue closed. Of course some pieces may be missing, but I think we should open other issues for those. Thank you!