Add test case for gh-761

This commit also ensures that a new local context is created, copying
the existing values. This avoids mutating the parent local context and
polluting it with local values.
This could cause unintended side effects on other child datafetchers.

Fixes gh-761

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

@@ -203,7 +203,7 @@ public class GraphQlObservationInstrumentation extends SimplePerformantInstrumen
 		}
 		else {
 			GraphQLContext localContext = dataFetcherLocalContext == null ?
-					GraphQLContext.getDefault() : dataFetcherLocalContext;
+					GraphQLContext.getDefault() : GraphQLContext.newContext().of(dataFetcherLocalContext).build();
 			return DataFetcherResult.newResult()
 					.data(value)
 					.localContext(localContext.put(ObservationThreadLocalAccessor.KEY, dataFetcherObservation))

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

@@ -16,6 +16,7 @@
 
 package org.springframework.graphql.observation;
 
+import graphql.GraphQLContext;
 import graphql.GraphqlErrorBuilder;
 import graphql.execution.DataFetcherResult;
 import graphql.schema.AsyncDataFetcher;
@@ -279,4 +280,39 @@ class GraphQlObservationInstrumentationTests {
 		ResponseHelper.forResponse(responseMono);
 	}
 
+	@Test
+	void shouldNotOverrideExistingLocalContext() {
+		
+		String document = """
+				{
+					bookById(id: 1) {
+						author {
+							firstName,
+							lastName
+						}
+					}
+				}
+				""";
+		DataFetcher<DataFetcherResult<Object>> bookDataFetcher = environment -> DataFetcherResult.newResult()
+				.data(BookSource.getBook(1L))
+				.localContext(GraphQLContext.newContext().of("test", "value").build())
+				.build();
+		DataFetcher<Author> authorDataFetcher = environment -> BookSource.getAuthor(101L);
+		DataFetcher<String> authorFirstNameDataFetcher = environment -> {
+			GraphQLContext context = environment.getLocalContext();
+			String value = context.get("test");
+			assertThat(value).isEqualTo("value");
+			return BookSource.getAuthor(101L).getFirstName();
+		};
+
+        ExecutionGraphQlRequest request = TestExecutionRequest.forDocument(document);
+		Mono<ExecutionGraphQlResponse> responseMono = graphQlSetup
+				.queryFetcher("bookById", bookDataFetcher)
+				.dataFetcher("Book", "author", authorDataFetcher)
+				.dataFetcher("Author", "firstName", authorFirstNameDataFetcher)
+				.toGraphQlService()
+				.execute(request);
+		ResponseHelper.forResponse(responseMono);
+	}
+
 }