Injects the span context into a `carrier` object.

        :param opentracing.span.SpanContext span: the SpanContext instance
        :param Any obj: Object to use as context

def inject(cls, span, obj):
        """ Injects the span context into a `carrier` object.

        :param opentracing.span.SpanContext span: the SpanContext instance
        :param Any obj: Object to use as context
        """
        obj.metadata['__parent-span__'] = dict()
        cls.inject_span(span, obj.metadata['__parent-span__'])