private void ensurePromptTokensDetailsIsMutable() {
      if (!((bitField0_ & 0x00000004) != 0)) {
        promptTokensDetails_ =
            new java.util.ArrayList<com.google.cloud.vertexai.api.ModalityTokenCount>(
                promptTokensDetails_);
        bitField0_ |= 0x00000004;
      }
    }
<pre>
The total number of billable characters counted across all instances from
the request.
</pre>

<code>int32 total_billable_characters = 2;</code>

@return This builder for chaining.
java-vertexai/proto-google-cloud-vertexai-v1/src/main/java/com/google/cloud/vertexai/api/CountTokensResponse.java