Convert a SpanContext object to B3 propagation headers.

        :type span_context:
            :class:`~opencensus.trace.span_context.SpanContext`
        :param span_context: SpanContext object.

        :rtype: dict
        :returns: B3 propagation headers.

def to_headers(self, span_context):
        """Convert a SpanContext object to B3 propagation headers.

        :type span_context:
            :class:`~opencensus.trace.span_context.SpanContext`
        :param span_context: SpanContext object.

        :rtype: dict
        :returns: B3 propagation headers.
        """

        if not span_context.span_id:
            span_id = INVALID_SPAN_ID
        else:
            span_id = span_context.span_id

        sampled = span_context.trace_options.enabled

        return {
            _TRACE_ID_KEY: span_context.trace_id,
            _SPAN_ID_KEY: span_id,
            _SAMPLED_KEY: '1' if sampled else '0'
        }