docs(performance): clarify that Dart network requests are not collected automatically (#18786)

Author: SelaseKay

docs/perf-mon/_custom-network-traces.md

@@ -7,9 +7,11 @@ your app.
 
 The
 [network request traces automatically collected by Performance Monitoring](/docs/perf-mon/network-traces)
-include most network requests for your app. However, some requests might not be
-reported or you might use a different library to make network requests. In these
-cases, you can use the Performance Monitoring API to manually instrument
+on Android and Apple platforms only include requests made by native code, such
+as native SDKs that your app depends on. Requests made from Dart, including
+requests made with `dart:io` `HttpClient`, `package:http` with its default
+client, or `Image.network`, are not collected automatically. To monitor these
+requests, use the Performance Monitoring API to manually instrument
 **_custom network request traces_**. Custom network request traces are only
 supported for Apple and Android apps.
 

docs/perf-mon/get-started.md

@@ -56,6 +56,13 @@ for individual Flutter screens. A single view controller encapsulates your
 entire Flutter application natively so the underlying native Firebase SDK is
 not aware of screen transitions.
 
+On Android and Apple platforms, automatic network request monitoring only covers
+requests made by native code, such as native SDKs that your app depends on.
+Requests made from Dart, including requests made with `dart:io` `HttpClient`,
+`package:http` with its default client, or `Image.network`, are not collected
+automatically. To monitor these requests, add
+[custom network request traces](/docs/perf-mon/custom-network-traces?platform=flutter).
+
 Note: When you add Performance Monitoring to your app, the Remote Config SDK is
 included as a dependency. If you already use Remote Config, you won't see
 any difference. However, if you're new to Remote Config, explore the
@@ -104,6 +111,9 @@ information from your app.
       * <code>Logging trace metric: <var>TRACE_NAME</var>, <var>FIREBASE_PERFORMANCE_CONSOLE_URL</var></code>
       * <code>Logging network request trace: <var>URL</var></code>
 
+    Network request trace logs appear for requests made from Dart only if you
+    have added custom network request traces for them.
+
 1. Click on the URL to view your data in the Firebase console. It may take a few
    moments for the data to update in the dashboard.
 
@@ -142,6 +152,9 @@ of the Firebase console.
   * Data for [HTTP/S network requests](/docs/perf-mon/network-traces) issued
     by your app
 
+* On Android and Apple platforms, monitor network requests made from Dart by
+  [instrumenting custom network request traces](/docs/perf-mon/custom-network-traces?platform=flutter).
+
 * [View, track, and filter](/docs/perf-mon/console) your
   performance data in the Firebase console.