Global propagator (#103)
Global propagator (#103)

Implement the [global
propagator](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/context/api-propagators.md#global-propagators)
api. This allows http text format propagator to be configured once and
used both globally in the application as well as 3rd party middleware.
open-telemetry/opentelemetry-rust,open-telemetry/opentelemetry-rust