Mark single arg ctor explicit in DynamicMessageFactory.

This is in line with best practices since implicit conversion in not necessary nor desired for this class.

PiperOrigin-RevId: 543446110

src/google/protobuf/dynamic_message.h

@@ -96,7 +96,10 @@ class PROTOBUF_EXPORT DynamicMessageFactory : public MessageFactory {
   //   parser to look for extensions in an alternate pool.  However, note that
   //   this is almost never what you want to do.  Almost all users should use
   //   the zero-arg constructor.
-  DynamicMessageFactory(const DescriptorPool* pool);
+#ifndef PROTOBUF_FUTURE_BREAKING_CHANGES
+  explicit
+#endif
+      DynamicMessageFactory(const DescriptorPool* pool);
   DynamicMessageFactory(const DynamicMessageFactory&) = delete;
   DynamicMessageFactory& operator=(const DynamicMessageFactory&) = delete;