* Remove TODO from ReadOnlySpan interface
* Remove the Tracer method from the ReadOnlySpan
This is not required by the specification nor the use of this interface.
* Remove IsRecording from the ReadOnlySpan interface
A read-only span value does not need to know if updates to it will be
recorded. It by definition cannot be updated so no point in
communicating if an update would be recorded.
* Document the ReadOnlySpan interface
* Rename messageEvent* to just event*
* Move the SpanSnapshot into its own file
* Update ReadOnlySpan interface with meta info methods
Add the DroppedAttributes, DroppedLinks, DroppedEvents, and
ChildSpanCount methods to the interface to return additional information
about the span not specified by the specification, but that we are
already providing.
* Add SpanStub to the sdk/trace/tracetest pkg
* Redefine ExportSpans of SpanExporter with ReadOnlySpan
* Rename SpanSnapshot to snapshot and purge docs
* Remove Snapshot method from snapshot type
This method is a hold-over from previous version of the ReadOnlySpan
interface is not needed.
* Update CHANGELOG with changes
