Fix observability instrumentation for CompletionStage

This commit fixes the observability instrumentation for data fetchers
when the return type is of `CompletionStage`. Prior to this commit, the
instrumentation of `CompletionStage` return values as
`DataFetcherResult` would wrap the asynchronous value twice.

Closes gh-676

Author: bclozel

spring-graphql/src/main/java/org/springframework/graphql/observation/GraphQlObservationInstrumentation.java

@@ -35,7 +35,7 @@
 import org.apache.commons.logging.LogFactory;
 import org.springframework.lang.Nullable;
 
-import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CompletionException;
 import java.util.concurrent.CompletionStage;
 
 /**
@@ -152,11 +152,10 @@ public DataFetcher<?> instrumentDataFetcher(DataFetcher<?> dataFetcher,
 							if (error != null) {
 								dataFetcherObservation.error(error);
 								dataFetcherObservation.stop();
-								return CompletableFuture.failedStage(error);
+								throw new CompletionException(error);
 							}
 							dataFetcherObservation.stop();
-							return CompletableFuture.completedStage(wrapAsDataFetcherResult(result, dataFetcherObservation));
-
+							return wrapAsDataFetcherResult(result, dataFetcherObservation);
 						});
 					}
 					else {

spring-graphql/src/test/java/org/springframework/graphql/observation/GraphQlObservationInstrumentationTests.java

@@ -68,6 +68,10 @@ void instrumentGraphQlRequestWhenSuccess(DataFetcher<?> dataFetcher) {
 				.toGraphQlService()
 				.execute(TestExecutionRequest.forDocument(document));
 		ResponseHelper response = ResponseHelper.forResponse(responseMono);
+
+		String name = response.rawValue("bookById.name");
+		assertThat(name).isEqualTo("Nineteen Eighty-Four");
+
 		TestObservationRegistryAssert.assertThat(this.observationRegistry).hasObservationWithNameEqualTo("graphql.request")
 				.that().hasLowCardinalityKeyValue("graphql.outcome", "SUCCESS")
 				.hasHighCardinalityKeyValueWithKey("graphql.execution.id");
@@ -167,8 +171,10 @@ void instrumentGraphQlRequestWhenDataFetchingFailure(DataFetcher<?> dataFetcher)
 				.toGraphQlService()
 				.execute(TestExecutionRequest.forDocument(document));
 		ResponseHelper response = ResponseHelper.forResponse(responseMono);
+		assertThat(response.error(0).message()).isEqualTo("Resolved error: book fetching failure");
+
 		TestObservationRegistryAssert.assertThat(this.observationRegistry).hasObservationWithNameEqualTo("graphql.request")
-				.that().hasLowCardinalityKeyValue("graphql.outcome", "SUCCESS")
+				.that().hasLowCardinalityKeyValue("graphql.outcome", "REQUEST_ERROR")
 				.hasHighCardinalityKeyValueWithKey("graphql.execution.id");
 
 		TestObservationRegistryAssert.assertThat(this.observationRegistry)