Factor out labelValue length check to abstract base class and small name fix

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=131902964

javatests/google/registry/monitoring/metrics/StoredMetricTest.java

@@ -69,7 +69,9 @@ public class StoredMetricTest {
             Boolean.class);
 
     thrown.expect(IllegalArgumentException.class);
-    thrown.expectMessage("The count of labelValues must be equal to");
+    thrown.expectMessage(
+        "The count of labelValues must be equal to the underlying Metric's count of labels.");
+
     dimensionalMetric.set(true, "foo");
   }