GH-3210: KafkaTemplate currentSpan tagging issue

Fixes: #3210

* When adding a tag to the current span during the sending of a kafka message
  using KafkaTemplate, the tag gets added to another span because KafkaTemplate
  doesn't open the scope for the started observation. Fixing this issue by wrapping
  the doSend method call in a proper observation scope.

**Auto-cherry-pick to `3.0.x`**

# Conflicts:
#	spring-kafka/src/test/java/org/springframework/kafka/support/micrometer/ObservationTests.java

spring-kafka/src/main/java/org/springframework/kafka/core/KafkaTemplate.java

@@ -770,7 +770,9 @@ private CompletableFuture<SendResult<K, V>> observeSend(final ProducerRecord<K,
 				this.observationRegistry);
 		try {
 			observation.start();
-			return doSend(producerRecord, observation);
+			try (Observation.Scope ignored = observation.openScope()) {
+				return doSend(producerRecord, observation);
+			}
 		}
 		catch (RuntimeException ex) {
 			// The error is added from org.apache.kafka.clients.producer.Callback