Add Kotlin samples for tracing

See gh-46699

Signed-off-by: Łukasz Jernaś <lukasz.jernas@allegro.com>

spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/actuator/micrometertracing/baggage/CreatingBaggage.kt

@@ -0,0 +1,30 @@
+/*
+ * 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.baggage
+
+import io.micrometer.tracing.Tracer
+import org.springframework.stereotype.Component
+
+@Component
+class CreatingBaggage(private val tracer: Tracer) {
+
+	fun doSomething() {
+		tracer.createBaggageInScope("baggage1", "value1").use {
+			// Business logic
+		}
+	}
+}

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

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+/*
+ * 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 val observationRegistry: ObservationRegistry) {
+
+	fun someOperation() {
+		Observation.createNotStarted("some-operation", observationRegistry)
+			.lowCardinalityKeyValue("some-tag", "some-value")
+			.observe {
+				// Business logic ...
+			}
+	}
+}

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

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+/*
+ * 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.apache.commons.logging.Log
+import org.apache.commons.logging.LogFactory
+import org.springframework.boot.autoconfigure.SpringBootApplication
+import org.springframework.boot.runApplication
+import org.springframework.web.bind.annotation.RequestMapping
+import org.springframework.web.bind.annotation.RestController
+
+@RestController
+@SpringBootApplication
+class MyApplication {
+
+	private val logger: Log = LogFactory.getLog(MyApplication::class.java)
+
+
+	@RequestMapping("/")
+	fun hello(): String {
+		logger.info("home() has been called")
+		return "Hello, World!"
+	}
+}
+
+fun main(args: Array<String>) {
+	runApplication<MyApplication>(*args)
+}