docs(node.js) Add instrumenting already loaded module

Provide additional information for instrumenting supported node.js modules that have been already loaded for cases where a user may have to delay instrumenting. An example of this might be asynchronously loading and decrypting api keys from a remote location.

src/content/docs/apm/agents/nodejs-agent/installation-configuration/install-nodejs-agent.mdx

@@ -70,6 +70,21 @@ To install the Node.js agent:
   ```
   node -r newrelic ./dist/server.js
   ```
+  If you are unable to `require('newrelic');` as the first line of your app's main module and you are unable to use the require flag as above (e.g. asynchronously loading api keys from a remote location during application bootstrapping), you may also add stock instrumentation to an already loaded [supported module](https://github.com/newrelic/node-newrelic/blob/0113eb5f0e707dc662a17d262a841503bab88841/lib/instrumentations.js#L6#L6) by using [`newrelic.instrumentLoadedModule`](https://docs.newrelic.com/docs/apm/agents/nodejs-agent/api-guides/nodejs-agent-api/#instrumentLoadedModule):
+
+  ```js
+  // load the agent
+  const newrelic = require('newrelic');
+
+  // module loaded before newrelic 
+  const expressModule = require('express');
+
+  // instrument express after the agent has been loaded
+  newrelic.instrumentLoadedModule(
+    'express',    // the module's name, as a string
+    expressModule // the module instance
+  );
+  ```
 </Callout>
 
 7. Optional: For additional [Node.js runtime-level statistics](/docs/agents/nodejs-agent/supported-features/node-vms-statistics-page), ensure the [`@newrelic/native-metrics` package is installed](/docs/agents/nodejs-agent/supported-features/node-vm-measurements).