private com.google.protobuf.SingleFieldBuilderV3<
            com.google.cloud.vertexai.api.CitationMetadata,
            com.google.cloud.vertexai.api.CitationMetadata.Builder,
            com.google.cloud.vertexai.api.CitationMetadataOrBuilder>
        getCitationMetadataFieldBuilder() {
      if (citationMetadataBuilder_ == null) {
        citationMetadataBuilder_ =
            new com.google.protobuf.SingleFieldBuilderV3<
                com.google.cloud.vertexai.api.CitationMetadata,
                com.google.cloud.vertexai.api.CitationMetadata.Builder,
                com.google.cloud.vertexai.api.CitationMetadataOrBuilder>(
                getCitationMetadata(), getParentForChildren(), isClean());
        citationMetadata_ = null;
      }
      return citationMetadataBuilder_;
    }
<pre>
Output only. Source attribution of the generated content.
</pre>

<code>
.google.cloud.vertexai.v1.CitationMetadata citation_metadata = 6 [(.google.api.field_behavior) = OUTPUT_ONLY];
</code>
java-vertexai/proto-google-cloud-vertexai-v1/src/main/java/com/google/cloud/vertexai/api/Candidate.java