@java.lang.Override
  public com.google.protobuf.ByteString getParentSpanIdBytes() {
    java.lang.Object ref = parentSpanId_;
    if (ref instanceof java.lang.String) {
      com.google.protobuf.ByteString b =
          com.google.protobuf.ByteString.copyFromUtf8((java.lang.String) ref);
      parentSpanId_ = b;
      return b;
    } else {
      return (com.google.protobuf.ByteString) ref;
    }
  }
<pre>
The `[SPAN_ID]` of this span's parent span. If this is a root span,
then this field must be empty.
</pre>

<code>string parent_span_id = 3;</code>

@return The bytes for parentSpanId.
java-trace/proto-google-cloud-trace-v2/src/main/java/com/google/devtools/cloudtrace/v2/Span.java