utilize esm proxy symbol

lib/instrumentation/fastify.js

@@ -11,6 +11,7 @@ const {
   buildMiddlewareSpecForMiddlewareFunction
 } = require('./fastify/spec-builders')
 const { MiddlewareMounterSpec } = require('../shim/specs')
+const symbols = require('../symbols')
 
 /**
  * These are the events that occur during a fastify
@@ -95,14 +96,11 @@ module.exports = function initialize(agent, fastify, moduleName, shim) {
    */
   const wrappedExport = shim.wrapExport(fastify, function wrapFastifyModule(shim, fn) {
     return function wrappedFastifyModule() {
-      let _fn = fn
-      if (Object.prototype.toString.call(fn) === '[object Object]') {
-        // The passed in `fn` is a proxy object returned by import-in-the-middle.
-        // So we need to pop the original export off it.
-        _fn = fn.fastify
+      if (fn[symbols.nrEsmProxy] === true) {
+        fn = fn.fastify
       }
       // call original function get get fastify object (which is singleton-ish)
-      const fastifyForWrapping = _fn.apply(this, arguments)
+      const fastifyForWrapping = fn.apply(this, arguments)
 
       setupRouteHandler(shim, fastifyForWrapping)