Ensure muzzle collects VirtualFields from static initializers (#13600)

Author: JonasKunz

muzzle/src/test/java/io/opentelemetry/javaagent/tooling/muzzle/ReferenceCollectorTest.java

@@ -363,6 +363,20 @@ public void shouldCollectMultipleVirtualFieldsForSingleClass() {
             entry(VirtualFieldTestClasses.Key1.class.getName(), State.class.getName()));
   }
 
+  @Test
+  public void shouldCollectVirtualFieldsFromStaticInitializers() {
+    ReferenceCollector collector = new ReferenceCollector(s -> s.endsWith("$Helper"));
+    collector.collectReferencesFromAdvice(
+        VirtualFieldTestClasses.VirtualFieldInStaticInitializerAdvice.class.getName());
+    collector.prune();
+
+    VirtualFieldMappings virtualFieldMappings = collector.getVirtualFieldMappings();
+    assertThat(virtualFieldMappings.entrySet())
+        .containsExactlyInAnyOrder(
+            entry(VirtualFieldTestClasses.Key1.class.getName(), Context.class.getName()),
+            entry(VirtualFieldTestClasses.Key2.class.getName(), Context.class.getName()));
+  }
+
   @ParameterizedTest(name = "{0}")
   @MethodSource
   public void shouldNotCollectVirtualFieldsForInvalidScenario(

muzzle/src/test/java/io/opentelemetry/javaagent/tooling/muzzle/VirtualFieldTestClasses.java

@@ -21,6 +21,24 @@ public static void advice() {
     }
   }
 
+  public static class VirtualFieldInStaticInitializerAdvice {
+
+    static final VirtualField<Key1, Context> FIELD_1 = VirtualField.find(Key1.class, Context.class);
+
+    public static class Helper {
+      private Helper() {}
+
+      static final VirtualField<Key2, Context> FIELD_2 =
+          VirtualField.find(Key2.class, Context.class);
+
+      public static void foo() {}
+    }
+
+    public static void advice() {
+      Helper.foo();
+    }
+  }
+
   public static class TwoVirtualFieldsInTheSameClassAdvice {
     public static void advice() {
       VirtualField.find(Key1.class, Context.class);