[analysis_server] Add some notes on how to use the CPU Profiler for the analysis server hosted by VS Code

Change-Id: I1af51746901d0eb579431c277b0cff204f025c42
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/397840
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Kenzie Davisson <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>
Reviewed-by: Samuel Rawlins <[email protected]>

Author: DanTup

pkg/analysis_server/doc/process/overview.md

@@ -10,3 +10,4 @@ The documentation covers the following topics:
 
 - [Implementing a new language feature](new_language_feature.md)
 - [Running/Debugging analysis_server from source in VS Code](running_in_vs_code.md)
+- [Profiling the analysis server hosted by VS Code](profiling_for_vs_code)

pkg/analysis_server/doc/process/profiling_for_vs_code.md

@@ -0,0 +1,40 @@
+# Profiling the analysis server hosted by VS Code
+
+The steps below show how to use the DevTools CPU profiler against the instance
+of the analysis server being hosted by VS Code. This allows profiling a short
+window of operations (such as typing in the editor or invoking code completion).
+
+- Open a project in VS Code that can be used to trigger the bad performance
+- Run the **Preferences: Open Workspace Settings (JSON)** command from the
+  command palette (`F1`) or open the `.vscode/settings.json` file
+- Add `"dart.analyzerVmServicePort": 8855` (the exact port number is not important
+  but you'll need it further down)
+- Save the settings file
+- A toast notification should appear in the bottom right warning you that the
+  analysis server's VM Service is enabled. This is a reminder that you should
+  remove the `dart.analyzerVmServicePort` setting when you are done.
+- Wait for initial analysis to complete (No "Analyzing..." or spinners in the
+  status bar)
+- Open a file and find a location where you can reproduce the bad performance
+- Run the **Dart: Open DevTools in Browser** command from the palette (`F1`)
+- In DevTools, enter `http://localhost:8855/` (using the port number from above)
+  and click **Connect**
+- Navigate to the **CPU Profiler** tab
+- Click **Enable Profiler**
+- In the isolate selector (bottom middle of the page), select the Isolate named
+  **main**
+- Click the **Start recording** button
+- Switch back to VS Code and perform the actions that cause bad performance
+- Back in DevTools, click the **Stop recording** button in the CPU Profiler
+
+Profiling information should be displayed automatically and can be exported
+using the export button on the top right corner. Exported information may
+contain paths and class/member names from your project.
+
+When you are done, don't forget to remove the VM Service port from your
+workspace settings:
+
+- Run the **Preferences: Open Workspace Settings (JSON)** command from the
+  command palette (`F1`) or open the `.vscode/settings.json` file
+- Remove the `"dart.analyzerVmServicePort": 8855` setting
+- Save the settings file