code: @ExperimentalApi("https://github.com/grpc/grpc-java/issues/1789")
    public static <T extends AbstractStub<T>> T captureMetadata(
        T stub,
        AtomicReference<Metadata> headersCapture,
        AtomicReference<Metadata> trailersCapture) {
      return stub.withInterceptors(
          newCaptureMetadataInterceptor(headersCapture, trailersCapture));
    }
docs: Captures the last received metadata for a stub. Useful for testing
  
  @param stub to capture for
  @param headersCapture to record the last received headers
  @param trailersCapture to record the last received trailers
  @return an implementation of the stub that allows to access the last received call's
  headers and trailers via {@code headersCapture} and {@code trailersCapture}.