fix TypeError in count_tokens & count_tokens_async (#123)

async def count_tokens(
        self,
        request: Optional[Union[generative_service.CountTokensRequest, dict]] = None,
        *,
        model: Optional[str] = None,
        contents: Optional[MutableSequence[content.Content]] = None,
        retry: OptionalRetry = gapic_v1.method.DEFAULT,
        timeout: Union[float, object] = gapic_v1.method.DEFAULT,
        metadata: Sequence[Tuple[str, str]] = (),
    if you don't use keyword args : model=model, contents=contents,  but position args, this will lead to : TypeError: count_tokens() takes from 1 to 2 positional arguments but 3 were given

google/generativeai/generative_models.py

@@ -275,13 +275,13 @@ def count_tokens(
         self, contents: content_types.ContentsType
     ) -> glm.CountTokensResponse:
         contents = content_types.to_contents(contents)
-        return self._client.count_tokens(self.model_name, contents)
+        return self._client.count_tokens(model=self.model_name, contents=contents)
 
     async def count_tokens_async(
         self, contents: content_types.ContentsType
     ) -> glm.CountTokensResponse:
         contents = content_types.to_contents(contents)
-        return await self._client.count_tokens(self.model_name, contents)
+        return await self._client.count_tokens(model=self.model_name, contents=contents)
     # fmt: on
 
     def start_chat(