[MINOR] test: fix NPE for test PartitionBalanceAssignmentStrategyTest (…

…#2322)

### What changes were proposed in this pull request?

Fix NPE for test PartitionBalanceAssignmentStrategyTest 

### Why are the changes needed?

It will NPE if The metrics have not register to MetricsSystem yet for UTs.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Existing UT.

coordinator/src/test/java/org/apache/uniffle/coordinator/strategy/assignment/BasicAssignmentStrategyTest.java

@@ -28,12 +28,15 @@
 import com.google.common.collect.Lists;
 import com.google.common.collect.Sets;
 import org.apache.hadoop.conf.Configuration;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 import org.apache.uniffle.common.PartitionRange;
 import org.apache.uniffle.coordinator.CoordinatorConf;
 import org.apache.uniffle.coordinator.ServerNode;
 import org.apache.uniffle.coordinator.SimpleClusterManager;
+import org.apache.uniffle.coordinator.metric.CoordinatorMetrics;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
@@ -45,6 +48,16 @@ public class BasicAssignmentStrategyTest {
   Set<String> tags = Sets.newHashSet("test");
   private int shuffleNodesMax = 7;
 
+  @BeforeEach
+  public void setUp() {
+    CoordinatorMetrics.register();
+  }
+
+  @AfterEach
+  public void clear() {
+    CoordinatorMetrics.clear();
+  }
+
   @Test
   public void testAssign() throws Exception {
     CoordinatorConf ssc = new CoordinatorConf();

coordinator/src/test/java/org/apache/uniffle/coordinator/strategy/assignment/PartitionBalanceAssignmentStrategyTest.java

@@ -30,11 +30,14 @@
 import com.google.common.collect.Sets;
 import com.google.common.util.concurrent.Uninterruptibles;
 import org.apache.hadoop.conf.Configuration;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 import org.apache.uniffle.coordinator.CoordinatorConf;
 import org.apache.uniffle.coordinator.ServerNode;
 import org.apache.uniffle.coordinator.SimpleClusterManager;
+import org.apache.uniffle.coordinator.metric.CoordinatorMetrics;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNull;
@@ -46,6 +49,16 @@ public class PartitionBalanceAssignmentStrategyTest {
   private int shuffleNodesMax = 5;
   private Set<String> tags = Sets.newHashSet("test");
 
+  @BeforeEach
+  public void setUp() {
+    CoordinatorMetrics.register();
+  }
+
+  @AfterEach
+  public void clear() {
+    CoordinatorMetrics.clear();
+  }
+
   @Test
   public void testAssign() throws Exception {
     CoordinatorConf ssc = new CoordinatorConf();