fix: Avoid unnuecessary exemplar allocations

prometheus-metrics-model/src/main/java/io/prometheus/metrics/model/snapshots/Exemplars.java

@@ -24,6 +24,10 @@ public class Exemplars implements Iterable<Exemplar> {
   private final List<Exemplar> exemplars;
 
   private Exemplars(Collection<Exemplar> exemplars) {
+    if (exemplars.isEmpty()) {
+      this.exemplars = Collections.emptyList();
+      return;
+    }
     List<Exemplar> copy = new ArrayList<>(exemplars.size());
     for (Exemplar exemplar : exemplars) {
       if (exemplar == null) {
@@ -41,7 +45,7 @@ private Exemplars(Collection<Exemplar> exemplars) {
    * @param exemplars a copy of the exemplars collection will be created.
    */
   public static Exemplars of(Collection<Exemplar> exemplars) {
-    return new Exemplars(exemplars);
+    return exemplars.isEmpty() ? EMPTY : new Exemplars(exemplars);
   }
 
   /**
@@ -51,7 +55,7 @@ public static Exemplars of(Collection<Exemplar> exemplars) {
    * @param exemplars a copy of the exemplars array will be created.
    */
   public static Exemplars of(Exemplar... exemplars) {
-    return new Exemplars(Arrays.asList(exemplars));
+    return exemplars.length == 0 ? EMPTY : new Exemplars(Arrays.asList(exemplars));
   }
 
   @Override

prometheus-metrics-model/src/test/java/io/prometheus/metrics/model/snapshots/ExemplarsTest.java

@@ -3,6 +3,7 @@
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 
+import java.util.Collections;
 import java.util.Iterator;
 import org.junit.jupiter.api.Test;
 
@@ -53,4 +54,22 @@ void testGet() {
 
     assertThat(exemplars.getLatest()).isSameAs(newest);
   }
+
+  @Test
+  void testEmptyReturnsSingleton() {
+    // Empty inputs short-circuit to the EMPTY singleton, avoiding allocations.
+    assertThat(Exemplars.of()).isSameAs(Exemplars.EMPTY);
+    assertThat(Exemplars.of(Collections.emptyList())).isSameAs(Exemplars.EMPTY);
+    assertThat(Exemplars.builder().build()).isSameAs(Exemplars.EMPTY);
+  }
+
+  @Test
+  void testEmptyIteratorIsSingleton() {
+    // Collections.emptyList().iterator() returns the JDK singleton empty iterator, so reading an
+    // empty Exemplars allocates no iterator.
+    assertThat(Exemplars.EMPTY.size()).isZero();
+    assertThat(Exemplars.EMPTY.iterator()).isSameAs(Collections.emptyIterator());
+    assertThat(Exemplars.EMPTY.get(0.0, Double.POSITIVE_INFINITY)).isNull();
+    assertThat(Exemplars.EMPTY.getLatest()).isNull();
+  }
 }