private com.google.protobuf.SingleFieldBuilderV3<
            com.google.protobuf.Struct,
            com.google.protobuf.Struct.Builder,
            com.google.protobuf.StructOrBuilder>
        getMetadataFieldBuilder() {
      if (metadataBuilder_ == null) {
        metadataBuilder_ =
            new com.google.protobuf.SingleFieldBuilderV3<
                com.google.protobuf.Struct,
                com.google.protobuf.Struct.Builder,
                com.google.protobuf.StructOrBuilder>(
                getMetadata(), getParentForChildren(), isClean());
        metadata_ = null;
      }
      return metadataBuilder_;
    }
<pre>
Input only. This field is for users to attach user managed metadata.
</pre>

<code>.google.protobuf.Struct metadata = 3 [(.google.api.field_behavior) = INPUT_ONLY];
</code>
java-visionai/proto-google-cloud-visionai-v1/src/main/java/com/google/cloud/visionai/v1/PacketHeader.java