Add micrometer tracing documentation

Closes gh-30658

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/actuator/micrometertracing/creatingspans/CustomObservation.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2012-2022 the original author or authors.
+ *
+ * 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 org.springframework.boot.docs.actuator.micrometertracing.creatingspans;
+
+import io.micrometer.observation.Observation;
+import io.micrometer.observation.ObservationRegistry;
+
+import org.springframework.stereotype.Component;
+
+@Component
+class CustomObservation {
+
+	private final ObservationRegistry observationRegistry;
+
+	CustomObservation(ObservationRegistry observationRegistry) {
+		this.observationRegistry = observationRegistry;
+	}
+
+	void someOperation() {
+		Observation observation = Observation.createNotStarted("some-operation", this.observationRegistry);
+		observation.lowCardinalityKeyValue("some-tag", "some-value");
+		observation.observe(() -> {
+			// Business logic ...
+		});
+	}
+
+}

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/actuator/micrometertracing/gettingstarted/MyApplication.java

@@ -0,0 +1,37 @@
+/*
+ * Copyright 2012-2022 the original author or authors.
+ *
+ * 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 org.springframework.boot.docs.actuator.micrometertracing.gettingstarted;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+@RestController
+@SpringBootApplication
+public class MyApplication {
+
+	@RequestMapping("/")
+	String home() {
+		return "Hello World!";
+	}
+
+	public static void main(String[] args) {
+		SpringApplication.run(MyApplication.class, args);
+	}
+
+}