Update default index type and metric type for MyScale vector store (#9353)

We update the default index type from `IVFFLAT` to `MSTG`, a new vector
type developed by MyScale.

libs/langchain/langchain/vectorstores/myscale.py

@@ -46,7 +46,7 @@ class MyScaleSettings(BaseSettings):
         table (str) : Table name to operate on.
                       Defaults to 'vector_table'.
         metric (str) : Metric to compute distance,
-                       supported are ('l2', 'cosine', 'ip'). Defaults to 'cosine'.
+                       supported are ('L2', 'Cosine', 'IP'). Defaults to 'Cosine'.
         column_map (Dict) : Column type map to project column name onto langchain
                             semantics. Must have keys: `text`, `id`, `vector`,
                             must be same size to number of columns. For example:
@@ -69,7 +69,7 @@ class MyScaleSettings(BaseSettings):
     username: Optional[str] = None
     password: Optional[str] = None
 
-    index_type: str = "IVFFLAT"
+    index_type: str = "MSTG"
     index_param: Optional[Dict[str, str]] = None
 
     column_map: Dict[str, str] = {
@@ -81,7 +81,7 @@ class MyScaleSettings(BaseSettings):
 
     database: str = "default"
     table: str = "langchain"
-    metric: str = "cosine"
+    metric: str = "Cosine"
 
     def __getitem__(self, item: str) -> Any:
         return getattr(self, item)