Nullify noLabels in StatefulMetric during clear() to prevent no-labels increase to stop working (#972)

Signed-off-by: Iacopo Pace <iaco.pace@gmail.com>
Co-authored-by: Iacopo Pace <iaco.pace@gmail.com>

Author: iaco86

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

@@ -114,6 +114,7 @@ public void remove(String... labelValues) {
      */
     public void clear() {
         data.clear();
+        noLabels = null;
     }
 
     protected abstract T newDataPoint();

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

@@ -60,5 +60,10 @@ public void testClearNoLabels() {
         // No labels is always present, but as no value has been observed after clear() the value should be 0.0
         Assert.assertEquals(1, counter.collect().getDataPoints().size());
         Assert.assertEquals(0.0, counter.collect().getDataPoints().get(0).getValue(), 0.0);
+
+        // Making inc() works correctly after clear()
+        counter.inc();
+        Assert.assertEquals(1, counter.collect().getDataPoints().size());
+        Assert.assertEquals(1.0, counter.collect().getDataPoints().get(0).getValue(), 0.0);
     }
 }