Add context API which follows https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/context/context.md. Related to #74. References: * [OpenTelemetry Python Implementation](https://github.com/open-telemetry/opentelemetry-python/tree/master/opentelemetry-api/src/opentelemetry/context)
Add context API which follows https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/context/context.md. Related to #74.
References: