Adds support for Kotlin Coroutines (#1977)

* Added Kotlin Coroutine support
* Added docs, polished code and added one more case for reactor context

tests/common/src/main/kotlin/org/springframework/cloud/sleuth/kotlin/SleuthCoroutinesApplicationTests.kt

@@ -0,0 +1,110 @@
+/*
+ * Copyright 2013-2021 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.cloud.sleuth.kotlin
+
+import kotlinx.coroutines.GlobalScope
+import kotlinx.coroutines.async
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.runBlocking
+import org.assertj.core.api.BDDAssertions.then
+import org.junit.jupiter.api.Test
+import org.slf4j.Logger
+import org.slf4j.LoggerFactory
+import org.springframework.beans.factory.annotation.Autowired
+import org.springframework.boot.autoconfigure.EnableAutoConfiguration
+import org.springframework.boot.test.context.TestConfiguration
+import org.springframework.boot.web.server.LocalServerPort
+import org.springframework.cloud.sleuth.Span
+import org.springframework.cloud.sleuth.Tracer
+import org.springframework.cloud.sleuth.instrument.kotlin.asContextElement
+import org.springframework.cloud.sleuth.instrument.kotlin.currentSpan
+import org.springframework.context.annotation.Bean
+import org.springframework.test.context.ContextConfiguration
+import org.springframework.web.bind.annotation.GetMapping
+import org.springframework.web.bind.annotation.RestController
+import org.springframework.web.client.RestTemplate
+import kotlin.coroutines.coroutineContext
+
+@ContextConfiguration(classes = [SleuthCoroutinesApplicationTests.ControllerTestConfig::class])
+abstract class SleuthCoroutinesApplicationTests {
+
+	@LocalServerPort
+	var serverPort: Int = 0
+
+	@Autowired
+	lateinit var tracer: Tracer
+
+	@Autowired
+	lateinit var webController: WebController
+
+	@Autowired
+	lateinit var restTemplate: RestTemplate
+
+	@Test
+	fun should_pass_tracing_context_within_coroutines(): Unit = runBlocking {
+		val nextSpan = tracer.nextSpan()
+		val traceId = nextSpan.context().traceId()
+		tracer.withSpan(nextSpan.start()).use { withSpan ->
+			val response = restTemplate.getForObject(applicationUrl(), String::class.java)
+			// span in the controller is set
+			then(webController.spanInController).isNotNull
+			then(webController.spanInController?.context()?.traceId()).isEqualTo(traceId)
+			// span in global scope launch is set
+			then(webController.spanInGlobalScopeLaunch).isNotNull
+			then(webController.spanInGlobalScopeLaunch?.context()?.traceId()).isEqualTo(traceId)
+			// span in global async scope is set
+			then(webController.spanInGlobalScopeAsync).isNotNull
+			then(webController.spanInGlobalScopeAsync?.context()?.traceId()).isEqualTo(traceId)
+			// trace id got propagated
+			then(response).isEqualTo(traceId)
+		}
+	}
+
+	private fun applicationUrl() = "http://localhost:$serverPort/hello"
+
+	@TestConfiguration
+	@EnableAutoConfiguration
+	internal class ControllerTestConfig {
+		@Bean
+		fun restTemplate(): RestTemplate = RestTemplate()
+
+		@Bean
+		fun webController(tracer: Tracer): WebController = WebController(tracer)
+	}
+}
+
+@RestController
+class WebController(val tracer: Tracer) {
+	val log: Logger = LoggerFactory.getLogger(this::class.java)
+	var spanInController: Span? = null
+	var spanInGlobalScopeLaunch: Span? = null
+	var spanInGlobalScopeAsync: Span? = null
+
+	@GetMapping("/hello")
+	suspend fun hello(): String? {
+		spanInController = coroutineContext.currentSpan()
+		GlobalScope.launch(tracer.asContextElement()) {
+			log.info("in Coroutines context (launch) - current span {}", coroutineContext.currentSpan())
+			spanInGlobalScopeLaunch = coroutineContext.currentSpan()
+		}
+		GlobalScope.async(tracer.asContextElement()) {
+			log.info("in Coroutines context (async)- current span {}", coroutineContext.currentSpan())
+			spanInGlobalScopeAsync = coroutineContext.currentSpan()
+		}.await()
+		return coroutineContext.currentSpan()?.context()?.traceId()
+	}
+}