[No QA] docs: describe performance metrics

contributingGuides/PERFORMANCE_METRICS.md

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+# Performance Metrics
+
+This project tracks various performance metrics to monitor and improve the application's efficiency and user experience.
+
+## Tracked Metrics
+
+The following table shows the key performance metrics that are being monitored in the application.
+
+Project is using Firebase for tracking these metrics. However, not all of them are sent there - some of them are only used internally by the Performance module.
+
+| Metric name | Sent to Firebase | Description | Start time | End time |
+|----------|----------|----------|----------|----------|
+| `_app_start`   | ✅ | The time between when the user opens the app and when the app is responsive.<br><br>**Platforms:** Android     | Starts when the app's `FirebasePerfProvider` `ContentProvider` completes its `onCreate` method.     | Stops when the first activity's `onResume()` method is called.     |
+| `js_loaded`    | ✅ | The time it takes for the JavaScript bundle to load. <br><br>**Platforms:** Android, iOS | **Android:** Starts in the `onCreate` method.<br><br>**iOS:** Starts in the AppDelegate's `didFinishLaunchingWithOptions` method.    | Stops at the first render of the app via native module on the JS side.     |
+| `_app_in_foreground`    | ✅ | The time when the app is running in the foreground and available to the user.<br><br>**Platforms:** Android, iOS     | **Android:** Starts when the first activity to reach the foreground has its `onResume()` method called. <br><br>**iOS:** Starts when the application receives the `UIApplicationDidBecomeActiveNotification` notification.   | **Android:** Stops when the last activity to leave the foreground has its `onStop()` method called. <br><br>**iOS:** Stops when it receives the `UIApplicationWillResignActiveNotification` notification.     |
+| `_app_in_background`    | ✅ | Time when the app is running in the background.<br><br>**Platforms:** Android, iOS    | **Android:** Starts when the last activity to leave the foreground has its `onStop()` method called. <br><br>**iOS:** Starts when the application receives the `UIApplicationWillResignActiveNotification` notification.   | **Android:** Stops when the first activity to reach the foreground has its `onResume()` method called. <br><br>**iOS:** Stops when it receives the `UIApplicationDidBecomeActiveNotification` notification.     |
+| `homepage_initial_render`   | ✅ | Time taken for the initial render of the app for a logged in user.<br><br>**Platforms:** All     | Starts with the  first render of the `AuthScreens` component.     | Stops once the `AuthScreens` component is mounted.     |
+| `sidebar_loaded`    | ❌ | Time taken for the Sidebar to load.<br><br>**Platforms:** All      | Starts when the Sidebar is mounted.     | Stops when the Splash Screen is hidden.     |
+| `calc_most_recent_last_modified_action`    | ✅ | Time taken to find the most recently modified report action or report.<br><br>**Platforms:** All      | Starts when the app reconnects to the network     | Ends when the app reconnects to the network and the most recent report action or report is found.     |
+| `search_render`   | ✅ | Time taken to render the Chat Finder page.<br><br>**Platforms:** All      | Starts when the Chat Finder icon in LHN is pressed.     | Stops when the list of available options is rendered for the first time.     |
+| `load_search_options`    | ✅ | Time taken to generate the list of options used in Chat Finder.<br><br>**Platforms:** All     | Starts when the `getSearchOptions` function is called.     | Stops when the list of available options is generated.     |
+| `search_filter_options`    | ✅ | Time taken to filter search options in Chat Finder by given search value.<br><br>**Platforms:** All     | Starts when user types something in the Chat Finder search input.     | Stops when the list of filtered options is generated.     |
+| `trie_initialization`   | ✅ | Time taken to build the emoji trie.<br><br>**Platforms:** All      | Starts when emoji trie begins to build.     | Stops when emoji trie building is complete.     |
+| `open_report`    | ❌ | Time taken to open a report.<br><br>**Platforms:** All      | Starts when the row in the `LHNOptionsList` is pressed.     | Stops when the `ReportActionsList` finishes laying out.     |
+| `switch_report`    | ✅ | Time taken to open report.<br><br>**Platforms:** All      | Starts when the chat in the LHN is pressed.      | Stops when the `ReportActionsList` finishes laying out.     |
+| `open_report_from_preview`   | ✅ | Time taken to open a report from preview.<br><br>(previously `switch_report_from_preview`)<br><br>**Platforms:** All     | Starts when the user presses the Report Preview.     | Stops when the `ReportActionsList` finishes laying out.     |
+| `switch_report_from_preview`   | ❌ | **[REMOVED]** Time taken to open a report from preview.      | Starts when the user presses the Report Preview.     | Stops when the `ReportActionsList` finishes laying out.     |
+| `chat_render`    | ✅ | Time taken to render  the Report screen.<br><br>**Platforms:** All      | Starts when the `ReportScreen` is being rendered for the first time.     | Stops once the `ReportScreen` component is mounted.     |
+| `report_initial_render`   | ❌ | Time taken to render the Report screen.<br><br>**Platforms:** All      | Starts when the first item is rendered in the `LHNOptionsList`.     | Stops when the `ReportActionsList` finishes laying out.     |
+| `open_report_thread`   | ✅ | Time taken to open a thread in a report.<br><br>**Platforms:** All      | Starts when user presses Report Action Item.     | Stops when the `ReportActionsList` finishes laying out.     |
+| `message_sent`    | ❌ | Time taken to send a message.<br><br>**Platforms:** All      | Starts when the new message is sent.     | Stops when the message is being rendered in the chat.     |
+
+## Documentation Maintenance
+
+To ensure this documentation remains accurate and useful, please adhere to the following guidelines when updating performance metrics:
+
+1. **New Metrics**: When a new metric is introduced in the codebase, add it to the table with all relevant details.
+
+2. **Metric Renaming**: If a metric is renamed, update the table entry. Mark the old name as deprecated and include a reference to the new name.
+
+3. **Metric Removal**: If a metric is no longer used, don't delete its entry. Instead, mark it as deprecated in the table and provide a brief explanation.
+
+4. **Code Location Changes**: If the placement of a metric in the code changes, update the "Start time" and "End time" columns to reflect the new location.
+
+
+## Additional Resources
+
+- [Firebase Documentation](https://firebase.google.com/docs)
+- [Firebase Performance Monitoring](https://firebase.google.com/docs/perf-mon)