<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _span_attrs_to_pb(span_attr, proto_type):
 """ Convert a span attribute dict to protobuf, including Links, Attributes, TimeEvents. Args: span_attr (dict):
 A dict that needs to be converted to protobuf. proto_type (str):
 The type of the Protobuf. Returns: An instance of the specified protobuf. """

    attr_pb = getattr(trace_pb2.Span, proto_type)()
    ParseDict(span_attr, attr_pb)
    return attr_pb