Remove memory leak from guaranteed unresolved/unrejected Promise.

I question whether or not we want to actually guarantee async execution of
this method and not give the opportunity to the user to actually make it
block if they'd like to, but we certainly should ensure that Promises should
not be created on every request and never resolve or reject, which seems
like a prime avenue for a DoS via memory leak.

Of course, these should be garbage collected eventually, but we can avoid
creating them that way.

packages/apollo-server-plugin-operation-registry/src/ApolloServerPluginOperationRegistry.ts

@@ -163,11 +163,11 @@ for observability purposes, but all operations will be permitted.`,
             requestContext.metrics.registeredOperation = true;
             return;
           } else {
-            if (options.onUnregisteredOperation) {
-              new Promise(() => {
-                if (options.onUnregisteredOperation) {
-                  options.onUnregisteredOperation(requestContext);
-                }
+            // If defined, this method should not block, whether async or not.
+            if (typeof options.onUnregisteredOperation === 'function') {
+              const onUnregisteredOperation = options.onUnregisteredOperation;
+              Promise.resolve().then(() => {
+                 onUnregisteredOperation(requestContext);
               });
             }
           }
@@ -234,11 +234,11 @@ for observability purposes, but all operations will be permitted.`,
           }
 
           if (shouldForbidOperation) {
-            if (options.onForbiddenOperation) {
-              new Promise(() => {
-                if (options.onForbiddenOperation) {
-                  options.onForbiddenOperation(requestContext);
-                }
+            // If defined, this method should not block, whether async or not.
+            if (typeof options.onForbiddenOperation === 'function') {
+              const onForbiddenOperation = options.onForbiddenOperation;
+              Promise.resolve().then(() => {
+                onForbiddenOperation(requestContext);
               });
             }
             if (!options.dryRun) {