Initialize distribution customizations even before config is applied

microprofile/metrics/pom.xml

@@ -130,6 +130,7 @@
                                 <exclude>**/TestDisabledMetrics.java</exclude>
                                 <exclude>**/TestSelectivelyDisabledMetrics.java</exclude>
                                 <exclude>**/TestConfigProcessing.java</exclude>
+                                <exclude>**/TestDistributionCustomizationsNoInit.java</exclude>
                             </excludes>
                             <systemPropertyVariables>
                                 <junit.jupiter.extensions.autodetection.enabled>true</junit.jupiter.extensions.autodetection.enabled>
@@ -196,6 +197,18 @@
                             </systemPropertyVariables>
                         </configuration>
                     </execution>
+                    <execution>
+                        <!-- Run separately so the normal MP container start-up does not happen. -->
+                        <id>test-dist-cust-with-no-init</id>
+                        <goals>
+                            <goal>test</goal>
+                        </goals>
+                        <configuration>
+                            <includes>
+                                <include>**/TestDistributionCustomizationsNoInit.java</include>
+                            </includes>
+                        </configuration>
+                    </execution>
                 </executions>
             </plugin>
             <plugin>

microprofile/metrics/src/main/java/io/helidon/microprofile/metrics/DistributionCustomizations.java

@@ -62,7 +62,7 @@ class DistributionCustomizations {
     private static final Duration DEFAULT_TIMER_MIN = Duration.ofMillis(5);
     private static final Duration DEFAULT_TIMER_MAX = Duration.ofSeconds(10);
 
-    private static DistributionCustomizations instance;
+    private static DistributionCustomizations instance = new DistributionCustomizations();
     private final List<Percentiles> percentileCustomizations;
     private final List<SummaryBuckets> summaryBucketCustomizations;
     private final List<TimerBuckets> timerBucketCustomizations;
@@ -82,6 +82,13 @@ private DistributionCustomizations(Config mpConfig) {
                                                                                                    Boolean::parseBoolean));
     }
 
+    private DistributionCustomizations() {
+        percentileCustomizations = List.of();
+        summaryBucketCustomizations = List.of();
+        timerBucketCustomizations = List.of();
+        summaryBucketDefaultCustomizations = List.of();
+    }
+
     static void init(Config mpConfig) {
         instance = new DistributionCustomizations(mpConfig);
     }

microprofile/metrics/src/test/java/io/helidon/microprofile/metrics/TestDistributionCustomizationsNoInit.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright (c) 2024 Oracle and/or its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.helidon.microprofile.metrics;
+
+import org.eclipse.microprofile.metrics.MetricRegistry;
+import org.eclipse.microprofile.metrics.Timer;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.notNullValue;
+
+class TestDistributionCustomizationsNoInit {
+
+    private static MetricRegistry metricRegistry;
+
+    @BeforeAll
+    static void initRegistry() {
+        metricRegistry = RegistryFactory.getInstance().getRegistry(MetricRegistry.APPLICATION_SCOPE);
+    }
+
+    @Test
+    void checkDistributionCustomizations() {
+        // Without the change in the main source, the following triggers an NPE because this test does not use @HelidonTest
+        // and therefore the normal metrics CDI extension initialization code--which sets up the distribution
+        // customizations--does not run. That means the configurable distribution customizations are never set, leading
+        // to the NPE.
+        Timer timer = metricRegistry.timer("testTimer");
+        assertThat("Timer", timer, notNullValue());
+    }
+}