Inherit local context from environment in instrumentation

Prior to this commit, a `DataFetcher` instrumented by the
`GraphQlObservationInstrumentation` would incorrectly overwrite the
local context when:

* the `DataFetcher` returns a value object (i.e. not a
  `DataFetcherResult`)
* the given `DatFetchingEnvironment` has an existing local context
with values

For this case, the instrumentation would create a new local context but
would not inherit from the existing local context.

See gh-774

Author: koenpunt

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

@@ -155,13 +155,13 @@ public DataFetcher<?> instrumentDataFetcher(DataFetcher<?> dataFetcher,
 								throw new CompletionException(error);
 							}
 							dataFetcherObservation.stop();
-							return wrapAsDataFetcherResult(result, dataFetcherObservation);
+							return wrapAsDataFetcherResult(result, dataFetcherObservation, environment.getLocalContext());
 						});
 					}
 					else {
 						observationContext.setValue(value);
 						dataFetcherObservation.stop();
-						return wrapAsDataFetcherResult(value, dataFetcherObservation);
+						return wrapAsDataFetcherResult(value, dataFetcherObservation, environment.getLocalContext());
 					}
 				}
 				catch (Throwable throwable) {
@@ -187,7 +187,8 @@ private static Observation getCurrentObservation(DataFetchingEnvironment environ
 		return currentObservation;
 	}
 
-	private static DataFetcherResult<?> wrapAsDataFetcherResult(Object value, Observation dataFetcherObservation) {
+	private static DataFetcherResult<?> wrapAsDataFetcherResult(Object value, Observation dataFetcherObservation,
+                    @Nullable GraphQLContext dataFetcherLocalContext) {
 		if (value instanceof DataFetcherResult<?> result) {
 			if (result.getLocalContext() == null) {
 				return result.transform(builder -> builder.localContext(GraphQLContext.newContext().of(ObservationThreadLocalAccessor.KEY, dataFetcherObservation).build()));
@@ -201,9 +202,11 @@ else if (result.getLocalContext() instanceof GraphQLContext) {
 			return result;
 		}
 		else {
+			GraphQLContext localContext = dataFetcherLocalContext == null ?
+					GraphQLContext.getDefault() : dataFetcherLocalContext;
 			return DataFetcherResult.newResult()
 					.data(value)
-					.localContext(GraphQLContext.newContext().of(ObservationThreadLocalAccessor.KEY, dataFetcherObservation).build())
+					.localContext(localContext.put(ObservationThreadLocalAccessor.KEY, dataFetcherObservation))
 					.build();
 		}