Fix flaky SlidingWindowTest#testRotate

Signed-off-by: Stephan Schroevers <[email protected]>

prometheus-metrics-core/src/test/java/io/prometheus/metrics/core/metrics/SlidingWindowTest.java

@@ -40,7 +40,7 @@ void assertValues(double... expectedValues) {
   private SlidingWindow<Observer> ringBuffer;
   private final long maxAgeSeconds = 30;
   private final int ageBuckets = 5;
-  private final long timeBetweenRotateMillis = maxAgeSeconds * 1000 / ageBuckets + 2;
+  private final long timeBetweenRotateMillis = maxAgeSeconds * 1000 / ageBuckets;
 
   @BeforeEach
   public void setUp() {