<DataCapture name="DC-Collect-ref-extra-attribute">
  <IgnoreUnresolvedVariables>false</IgnoreUnresolvedVariables>
  <ThrowExceptionOnLimit>false</ThrowExceptionOnLimit>

  <Capture>
    <DataCollector>dc_req_headers_length</DataCollector>
    <!-- unsupported attribute -->
    <Collect ref="computed-headers-length" default="0" extra-attr='hello'/>
  </Capture>
</DataCapture>
