Global observation registry

This change adds a static ObservationRegistry.global similar to the Metrics.globalRegistry. For the same reasons, for use in places where dependency injection is not desired.

micrometer-observation/src/main/java/io/micrometer/observation/DelegatingObservationRegistry.java

@@ -0,0 +1,57 @@
+/*
+ * Copyright 2022 VMware, Inc.
+ *
+ * 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
+ *
+ * https://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.micrometer.observation;
+
+/**
+ * Delegating {@link ObservationRegistry}.
+ *
+ * @author Christian Fredriksson
+ * @since 1.14.2
+ */
+class DelegatingObservationRegistry implements ObservationRegistry {
+
+    private ObservationRegistry delegate = ObservationRegistry.NOOP;
+
+    @Override
+    public Observation getCurrentObservation() {
+        return delegate.getCurrentObservation();
+    }
+
+    @Override
+    public Observation.Scope getCurrentObservationScope() {
+        return delegate.getCurrentObservationScope();
+    }
+
+    @Override
+    public void setCurrentObservationScope(Observation.Scope current) {
+        delegate.setCurrentObservationScope(current);
+    }
+
+    @Override
+    public ObservationConfig observationConfig() {
+        return delegate.observationConfig();
+    }
+
+    @Override
+    public boolean isNoop() {
+        return delegate.isNoop();
+    }
+
+    void set(ObservationRegistry delegate) {
+        this.delegate = delegate != null ? delegate : ObservationRegistry.NOOP;
+    }
+
+}

micrometer-observation/src/main/java/io/micrometer/observation/ObservationRegistry.java

@@ -42,6 +42,19 @@ static ObservationRegistry create() {
         return new SimpleObservationRegistry();
     }
 
+    /**
+     * Creates an instance of {@link ObservationRegistry}.
+     * @return {@link ObservationRegistry} instance
+     */
+    static void setGlobal(ObservationRegistry registry) {
+        ((DelegatingObservationRegistry) global).set(registry);
+    }
+
+    /**
+     * Global {@link ObservationRegistry}.
+     */
+    ObservationRegistry global = new DelegatingObservationRegistry();
+
     /**
      * No-op implementation of {@link ObservationRegistry}.
      */