docs: log warning to users that Part.from_video_metadata will be deprecated.

PiperOrigin-RevId: 738977752

Author: yyyu-google

google/genai/types.py

@@ -623,6 +623,11 @@ def from_function_response(
 
   @classmethod
   def from_video_metadata(cls, *, start_offset: str, end_offset: str) -> 'Part':
+    logger.warning("""Part.from_video_metadata will be deprecated soon.
+           Because a Part instance needs to include at least one of the fields:
+           text, file_data, inline_data, function_call, function_response, executable_code or code_execution_result.
+           A Part instance contains only video_metadata is not a valid Part.
+        """)
     video_metadata = VideoMetadata(
         end_offset=end_offset, start_offset=start_offset
     )