Gauge lookup infinite loop when CompositeRegistry is still empty

Follows-up on vert-x3#143

Signed-off-by: Thomas Segismont <tsegismont@gmail.com>

src/main/java/io/vertx/micrometer/impl/meters/Gauges.java

@@ -18,6 +18,8 @@
 package io.vertx.micrometer.impl.meters;
 
 import io.micrometer.core.instrument.*;
+import io.micrometer.core.instrument.composite.CompositeMeterRegistry;
+import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
 import io.vertx.micrometer.Label;
 import io.vertx.micrometer.impl.Labels;
 
@@ -66,6 +68,16 @@ public T get(Iterable<Tag> customTags, String... values) {
       .description(description)
       .tags(tags)
       .register(registry);
+    SimpleMeterRegistry smr = null;
+    if (registry instanceof CompositeMeterRegistry) {
+      CompositeMeterRegistry cmr = (CompositeMeterRegistry) registry;
+      if (cmr.getRegistries().isEmpty()) {
+        // If the composite meter registry has no children, the ToDoubleFunc will not be invoked
+        // So we temporarily add this SimpleMeterRegistry to deceive Micrometer
+        smr = new SimpleMeterRegistry();
+        cmr.add(smr);
+      }
+    }
     Meter.Id gaugeId = gauge.getId();
     Object res;
     for (; ; ) {
@@ -80,6 +92,10 @@ public T get(Iterable<Tag> customTags, String... values) {
         break;
       }
     }
+    if (smr != null) {
+      CompositeMeterRegistry cmr = (CompositeMeterRegistry) registry;
+      cmr.remove(smr);
+    }
     return (T) res;
   }
 

src/test/java/io/vertx/micrometer/EmptyCompositeMeterRegistryTest.java

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2022 Red Hat, Inc. and/or its affiliates
+ * and other contributors as indicated by the @author tags.
+ *
+ * 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.vertx.micrometer;
+
+import io.micrometer.core.instrument.composite.CompositeMeterRegistry;
+import io.vertx.ext.unit.TestContext;
+import io.vertx.ext.unit.junit.VertxUnitRunner;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+@RunWith(VertxUnitRunner.class)
+public class EmptyCompositeMeterRegistryTest extends MicrometerMetricsTestBase {
+
+  @Override
+  protected MicrometerMetricsOptions metricOptions() {
+    CompositeMeterRegistry emptyCompositeRegistry = new CompositeMeterRegistry();
+    return new MicrometerMetricsOptions()
+      .setRegistryName(registryName)
+      .setMicrometerRegistry(emptyCompositeRegistry)
+      .setEnabled(true);
+  }
+
+  @Test
+  public void simplyStarts(TestContext ctx) {
+    vertx = vertx(ctx);
+
+    // If the task is executed then the gauge lookup succedeed
+    vertx.executeBlocking(prom -> prom.complete(), ctx.asyncAssertSuccess());
+  }
+}