@Test
  public void testFlush() {
    Counter flushesCounter = mock(Counter.class);
    when(this.keyValueStoreMetrics.flushes()).thenReturn(flushesCounter);
    this.inMemoryKeyValueStore.flush();
    verify(flushesCounter).inc();
  }
The length of the result of {@link #value(int)} will always be the same, so this can be used as the length for any
key produced by {@link #value(int)}.
samza-kv-inmemory/src/test/java/org/apache/samza/storage/kv/inmemory/TestInMemoryKeyValueStore.java