* Add global propagator testing and other fixes
Use the testing framework from #1259 to test the added global
implementation of the TextMapPropagator.
Add tests for the internal state of the global package. Ensure the
behavior of setting the global for an already set value panics.
Fix oteltest `SetN` method check on the TextMapCarrier.
* Fix lint
* Add oteltest text map propagator and carrier
* Add changes to CHANGELOG
* Add PR number to CHANGELOG
* Add test for empty newState
Gotta farm that codecov