We need to replace the [RuntimeContext::context_handler_](https://github.com/open-telemetry/opentelemetry-cpp/blob/master/api/include/opentelemetry/context/runtime_context.h#L69), as we shouldn't have static globals in the API. A solution similar to the [TracerProvider](https://github.com/open-telemetry/opentelemetry-cpp/blob/master/api/include/opentelemetry/trace/provider.h#L24) singleton should be implemented.
We need to replace the RuntimeContext::context_handler_, as we shouldn't have static globals in the API.
A solution similar to the TracerProvider singleton should be implemented.