private com.google.protobuf.MapField<java.lang.String, java.lang.String> internalGetLabels() {
    if (labels_ == null) {
      return com.google.protobuf.MapField.emptyMapField(LabelsDefaultEntryHolder.defaultEntry);
    }
    return labels_;
  }
<pre>
Optional. ID of the parent span, if any.
</pre>

<code>fixed64 parent_span_id = 6 [(.google.api.field_behavior) = OPTIONAL];</code>

@return The parentSpanId.
java-trace/proto-google-cloud-trace-v1/src/main/java/com/google/devtools/cloudtrace/v1/TraceSpan.java