feat(extend-app-start): Add public Sentry app start extension API

Static bridge delegating to options.getAppStartExtender(): extendAppStart(),
finishAppStart(), getExtendedAppStartSpan(). No-op when the SDK is disabled or the
platform provides no app start extender. Completes the extend app start feature (4/4).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: buenaflor

sentry/api/sentry.api

@@ -2769,7 +2769,9 @@ public final class io/sentry/Sentry {
 	public static fun continueTrace (Ljava/lang/String;Ljava/util/List;)Lio/sentry/TransactionContext;
 	public static fun distribution ()Lio/sentry/IDistributionApi;
 	public static fun endSession ()V
+	public static fun extendAppStart ()V
 	public static fun feedback ()Lio/sentry/IFeedbackApi;
+	public static fun finishExtendedAppStart ()V
 	public static fun flush (J)V
 	public static fun forkedCurrentScope (Ljava/lang/String;)Lio/sentry/IScopes;
 	public static fun forkedRootScopes (Ljava/lang/String;)Lio/sentry/IScopes;
@@ -2778,6 +2780,7 @@ public final class io/sentry/Sentry {
 	public static fun getCurrentHub ()Lio/sentry/IHub;
 	public static fun getCurrentScopes ()Lio/sentry/IScopes;
 	public static fun getCurrentScopes (Z)Lio/sentry/IScopes;
+	public static fun getExtendedAppStartSpan ()Lio/sentry/ISpan;
 	public static fun getGlobalScope ()Lio/sentry/IScope;
 	public static fun getLastEventId ()Lio/sentry/protocol/SentryId;
 	public static fun getSpan ()Lio/sentry/ISpan;

sentry/src/main/java/io/sentry/Sentry.java

@@ -1277,6 +1277,38 @@ public static void reportFullyDisplayed() {
     getCurrentScopes().reportFullyDisplayed();
   }
 
+  /**
+   * Begins extending the app start so launch-time work done after {@code Application.onCreate}
+   * (e.g. loading remote config before the first screen) is included in the app start measurement.
+   *
+   * <p>Intended to be called from {@code Application.onCreate} right after {@code
+   * SentryAndroid.init}. Only effective on Android with standalone app start tracing enabled;
+   * otherwise it is a no-op. Also no-ops if the app start already finished, none is in progress, or
+   * it was already extended (first call wins). Call {@link #finishExtendedAppStart()} once the
+   * extra work is done; if it is never called, the app start transaction is finished by its
+   * deadline and no extended measurement is reported.
+   */
+  public static void extendAppStart() {
+    getCurrentScopes().getOptions().getAppStartExtender().extendAppStart();
+  }
+
+  /**
+   * Finishes the app start extension started by {@link #extendAppStart()}, allowing the app start
+   * transaction to complete. No-ops if the app start was not extended or this was already called.
+   */
+  public static void finishExtendedAppStart() {
+    getCurrentScopes().getOptions().getAppStartExtender().finishExtendedAppStart();
+  }
+
+  /**
+   * Returns the active extended app start span, to attach child spans for the launch-time work
+   * being measured, or a no-op span when no extension is active (e.g. {@link #extendAppStart()} was
+   * not called, the app start window already passed, or standalone app start tracing is disabled).
+   */
+  public static @NotNull ISpan getExtendedAppStartSpan() {
+    return getCurrentScopes().getOptions().getAppStartExtender().getExtendedAppStartSpan();
+  }
+
   /**
    * Configuration options callback
    *

sentry/src/test/java/io/sentry/SentryTest.kt

@@ -1769,4 +1769,53 @@ class SentryTest {
     val scopes = Sentry.getCurrentScopes()
     assertFalse(scopes.isNoOp)
   }
+
+  // region app start extension
+
+  private fun initWithExtender(extender: IAppStartExtender) {
+    initForTest {
+      it.dsn = dsn
+      it.appStartExtender = extender
+    }
+  }
+
+  @Test
+  fun `extendAppStart delegates to the app start extender`() {
+    val extender = mock<IAppStartExtender>()
+    initWithExtender(extender)
+
+    Sentry.extendAppStart()
+
+    verify(extender).extendAppStart()
+  }
+
+  @Test
+  fun `finishExtendedAppStart delegates to the app start extender`() {
+    val extender = mock<IAppStartExtender>()
+    initWithExtender(extender)
+
+    Sentry.finishExtendedAppStart()
+
+    verify(extender).finishExtendedAppStart()
+  }
+
+  @Test
+  fun `getExtendedAppStartSpan delegates to the app start extender`() {
+    val span = mock<ISpan>()
+    val extender = mock<IAppStartExtender>()
+    whenever(extender.extendedAppStartSpan).thenReturn(span)
+    initWithExtender(extender)
+
+    assertSame(span, Sentry.getExtendedAppStartSpan())
+  }
+
+  @Test
+  fun `app start extension api is a no-op when the SDK is disabled`() {
+    // beforeTest called Sentry.close(), so the current scopes are NoOp.
+    Sentry.extendAppStart()
+    Sentry.finishExtendedAppStart()
+    assertTrue(Sentry.getExtendedAppStartSpan().isNoOp)
+  }
+
+  // endregion
 }