public static <K, V> PerKeySketch<K, V> perKey() {
    return PerKeySketch.<K, V>builder().build();
  }
@Test
  public void perKeyDefault() {
    PCollection<Long> stream = tp.apply(Create.of(smallStream));
    PCollection<Sketch<Long>> sketch =
        stream.apply(WithKeys.of(1)).apply(SketchFrequencies.perKey()).apply(Values.create());

    Coder<Long> coder = stream.getCoder();

    PAssert.thatSingleton("Verify number of hits", sketch)
        .satisfies(new VerifyStreamFrequencies<>(coder, distinctElems, frequencies));

    tp.run();
  }