Null labels means label is not present (#2766)

Signed-off-by: Bogdan Drutu <bogdandrutu@gmail.com>

Author: bogdandrutu

api/metrics/src/main/java/io/opentelemetry/api/metrics/common/ArrayBackedLabels.java

@@ -22,7 +22,7 @@ static Labels empty() {
   }
 
   static Labels sortAndFilterToLabels(Object... data) {
-    return new ArrayBackedLabels(sortAndFilter(data, /* filterNullValues= */ false));
+    return new ArrayBackedLabels(sortAndFilter(data, /* filterNullValues= */ true));
   }
 
   @Override

api/metrics/src/test/java/io/opentelemetry/api/metrics/common/LabelsTest.java

@@ -74,6 +74,23 @@ void orderIndependentEquality() {
     assertThat(one).isEqualTo(two);
   }
 
+  @Test
+  void nullValueEquivalentWithMissing() {
+    Labels one =
+        Labels.of(
+            "key3", "value3",
+            "key4", null,
+            "key1", "value1",
+            "key2", "value2");
+    Labels two =
+        Labels.of(
+            "key2", "value2",
+            "key3", "value3",
+            "key1", "value1");
+
+    assertThat(one).isEqualTo(two);
+  }
+
   @Test
   void deduplication() {
     Labels one =