def encode_pb_list(pb_spans):
pb_list = zipkin_pb2.ListOfSpans()<EOL>pb_list.spans.extend(pb_spans)<EOL>return pb_list.SerializeToString()<EOL>
Encode list of protobuf Spans to binary.

    :param pb_spans: list of protobuf Spans.
    :type pb_spans: list of zipkin_pb2.Span
    :return: encoded list.
    :rtype: bytes