Fix flaky SlidingWindowTest (#1242)

Fixes issue #956.

Signed-off-by: Stephan Schroevers <stephan.schroevers@teampicnic.com>

Author: Stephan202

prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/metrics/SlidingWindow.java

@@ -26,7 +26,7 @@ public class SlidingWindow<T> {
   private int currentBucket;
   private long lastRotateTimestampMillis;
   private final long durationBetweenRotatesMillis;
-  LongSupplier currentTimeMillis = System::currentTimeMillis; // to be replaced in unit tests
+  private final LongSupplier currentTimeMillis;
 
   /**
    * Example: If the {@code maxAgeSeconds} is 60 and {@code ageBuckets} is 3, then 3 instances of
@@ -39,13 +39,24 @@ public class SlidingWindow<T> {
    * @param maxAgeSeconds after this amount of time an instance of T gets evicted.
    * @param ageBuckets number of age buckets.
    */
-  @SuppressWarnings("unchecked")
   public SlidingWindow(
       Class<T> clazz,
       Supplier<T> constructor,
       ObjDoubleConsumer<T> observeFunction,
       long maxAgeSeconds,
       int ageBuckets) {
+    this(clazz, constructor, observeFunction, maxAgeSeconds, ageBuckets, System::currentTimeMillis);
+  }
+
+  // VisibleForTesting
+  @SuppressWarnings("unchecked")
+  SlidingWindow(
+      Class<T> clazz,
+      Supplier<T> constructor,
+      ObjDoubleConsumer<T> observeFunction,
+      long maxAgeSeconds,
+      int ageBuckets,
+      LongSupplier currentTimeMillis) {
     this.constructor = constructor;
     this.observeFunction = observeFunction;
     this.ringBuffer = (T[]) Array.newInstance(clazz, ageBuckets);
@@ -55,6 +66,7 @@ public SlidingWindow(
     this.currentBucket = 0;
     this.lastRotateTimestampMillis = currentTimeMillis.getAsLong();
     this.durationBetweenRotatesMillis = TimeUnit.SECONDS.toMillis(maxAgeSeconds) / ageBuckets;
+    this.currentTimeMillis = currentTimeMillis;
   }
 
   /** Get the currently active instance of {@code T}. */

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

@@ -48,8 +48,12 @@ public void setUp() {
     currentTimeMillis.set(startTime);
     ringBuffer =
         new SlidingWindow<>(
-            Observer.class, Observer::new, Observer::observe, maxAgeSeconds, ageBuckets);
-    ringBuffer.currentTimeMillis = currentTimeMillis::get;
+            Observer.class,
+            Observer::new,
+            Observer::observe,
+            maxAgeSeconds,
+            ageBuckets,
+            currentTimeMillis::get);
   }
 
   @Test