Add getExtensionRegistry() getter to ProtoLiteUtils and ProtoUtils (#3836)

The ProtoLiteUtils.setExtensionRegistry() setter was added in
gRPC 1.0.0, but there was no corresponding getter. This made it
impossible for the reflection service or other components to
discover the extensions that were registered via the global
ExtensionRegistry.

Add getExtensionRegistry() to both ProtoLiteUtils (returns
ExtensionRegistryLite) and ProtoUtils (returns ExtensionRegistry
with a cast) so that the global registry can be read back.

Issue: #3836

Author: waterWang

protobuf-lite/src/main/java/io/grpc/protobuf/lite/ProtoLiteUtils.java

@@ -77,6 +77,15 @@ public static void setExtensionRegistry(ExtensionRegistryLite newRegistry) {
     globalRegistry = checkNotNull(newRegistry, "newRegistry");
   }
 
+  /**
+   * Returns the global extension registry.
+   *
+   * @since 1.62.0
+   */
+  public static ExtensionRegistryLite getExtensionRegistry() {
+    return globalRegistry;
+  }
+
   /**
    * Creates a {@link Marshaller} for protos of the same type as {@code defaultInstance}.
    *

protobuf/src/main/java/io/grpc/protobuf/ProtoUtils.java

@@ -46,6 +46,15 @@ public static void setExtensionRegistry(ExtensionRegistry registry) {
     ProtoLiteUtils.setExtensionRegistry(registry);
   }
 
+  /**
+   * Returns the global extension registry.
+   *
+   * @since 1.62.0
+   */
+  public static ExtensionRegistry getExtensionRegistry() {
+    return (ExtensionRegistry) ProtoLiteUtils.getExtensionRegistry();
+  }
+
   /**
    * Create a {@link Marshaller} for protos of the same type as {@code defaultInstance}.
    *