Remove internal public validateLabelPairs, move to the class that uses it (#2215)

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

api/internal/src/main/java/io/opentelemetry/api/internal/Utils.java

@@ -5,7 +5,6 @@
 
 package io.opentelemetry.api.internal;
 
-import java.util.Objects;
 import javax.annotation.concurrent.Immutable;
 
 /** General internal utility methods. */
@@ -26,23 +25,4 @@ public final class Utils {
       throw new IllegalArgumentException(errorMessage);
     }
   }
-
-  /**
-   * Validates that the array of Strings is 1) even in length, and 2) they can be formed into valid
-   * pairs where the first item in the pair is not null.
-   *
-   * <p>TODO: write unit tests for this method.
-   *
-   * @param keyValuePairs The String[] to validate for correctness.
-   * @throws IllegalArgumentException if any of the preconditions are violated.
-   */
-  public static void validateLabelPairs(String[] keyValuePairs) {
-    checkArgument(
-        keyValuePairs.length % 2 == 0,
-        "You must provide an even number of key/value pair arguments.");
-    for (int i = 0; i < keyValuePairs.length; i += 2) {
-      String key = keyValuePairs[i];
-      Objects.requireNonNull(key, "You cannot provide null keys for label creation.");
-    }
-  }
 }

api/metrics/src/main/java/io/opentelemetry/api/metrics/DefaultMeter.java

@@ -114,7 +114,7 @@ final class DefaultMeter implements Meter {
 
   @Override
   public BatchRecorder newBatchRecorder(String... keyValuePairs) {
-    Utils.validateLabelPairs(keyValuePairs);
+    validateLabelPairs(keyValuePairs);
     return NoopBatchRecorder.INSTANCE;
   }
 
@@ -676,4 +676,21 @@ final class DefaultMeter implements Meter {
 
     protected abstract B getThis();
   }
+
+  /**
+   * Validates that the array of Strings is 1) even in length, and 2) they can be formed into valid
+   * pairs where the first item in the pair is not null.
+   *
+   * @param keyValuePairs The String[] to validate for correctness.
+   * @throws IllegalArgumentException if any of the preconditions are violated.
+   */
+  private static void validateLabelPairs(String[] keyValuePairs) {
+    Utils.checkArgument(
+        keyValuePairs.length % 2 == 0,
+        "You must provide an even number of key/value pair arguments.");
+    for (int i = 0; i < keyValuePairs.length; i += 2) {
+      String key = keyValuePairs[i];
+      Objects.requireNonNull(key, "You cannot provide null keys for label creation.");
+    }
+  }
 }

api/metrics/src/test/java/io/opentelemetry/api/metrics/BatchRecorderTest.java

@@ -13,10 +13,16 @@ class BatchRecorderTest {
   private static final Meter meter = Meter.getDefault();
 
   @Test
-  void testNewBatchRecorder_badLabelSet() {
+  void testNewBatchRecorder_WrongNumberOfLabels() {
     assertThrows(IllegalArgumentException.class, () -> meter.newBatchRecorder("key"), "key/value");
   }
 
+  @Test
+  void testNewBatchRecorder_NullLabelKey() {
+    assertThrows(
+        NullPointerException.class, () -> meter.newBatchRecorder(null, "value"), "null keys");
+  }
+
   @Test
   void preventNull_MeasureLong() {
     assertThrows(