Add firestore_client param to FirestoreChatMessageHistory if caller already has one; also lets them specify GCP project, etc. (#8601)

Existing implementation requires that you install `firebase-admin`
package, and prevents you from using an existing Firestore client
instance if available.

This adds optional `firestore_client` param to
`FirestoreChatMessageHistory`, so users can just use their existing
client/settings. If not passed, existing logic executes to initialize a
`firestore_client`.

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/langchain/langchain/memory/chat_message_histories/firestore.py

@@ -12,39 +12,10 @@ from langchain.schema.messages import BaseMessage, messages_from_dict, messages_
 logger = logging.getLogger(__name__)
 
 if TYPE_CHECKING:
-    from google.cloud.firestore import DocumentReference
+    from google.cloud.firestore import Client, DocumentReference
 
 
-class FirestoreChatMessageHistory(BaseChatMessageHistory):
+def _get_firestore_client() -> Client:
-    """Chat message history backed by Google Firestore."""
-
-    def __init__(
-        self,
-        collection_name: str,
-        session_id: str,
-        user_id: str,
-    ):
-        """
-        Initialize a new instance of the FirestoreChatMessageHistory class.
-
-        :param collection_name: The name of the collection to use.
-        :param session_id: The session ID for the chat..
-        :param user_id: The user ID for the chat.
-        """
-        self.collection_name = collection_name
-        self.session_id = session_id
-        self.user_id = user_id
-
-        self._document: Optional[DocumentReference] = None
-        self.messages: List[BaseMessage] = []
-
-        self.prepare_firestore()
-
-    def prepare_firestore(self) -> None:
-        """Prepare the Firestore client.
-
-        Use this function to make sure your database is ready.
-        """
     try:
         import firebase_admin
         from firebase_admin import firestore
@@ -61,7 +32,39 @@ class FirestoreChatMessageHistory(BaseChatMessageHistory):
         logger.debug("Initializing Firebase app: %s", e)
         firebase_admin.initialize_app()
 
-        self.firestore_client = firestore.client()
+    return firestore.client()
+
+
+class FirestoreChatMessageHistory(BaseChatMessageHistory):
+    """Chat message history backed by Google Firestore."""
+
+    def __init__(
+        self,
+        collection_name: str,
+        session_id: str,
+        user_id: str,
+        firestore_client: Optional[Client] = None,
+    ):
+        """
+        Initialize a new instance of the FirestoreChatMessageHistory class.
+
+        :param collection_name: The name of the collection to use.
+        :param session_id: The session ID for the chat..
+        :param user_id: The user ID for the chat.
+        """
+        self.collection_name = collection_name
+        self.session_id = session_id
+        self.user_id = user_id
+        self._document: Optional[DocumentReference] = None
+        self.messages: List[BaseMessage] = []
+        self.firestore_client = firestore_client or _get_firestore_client()
+        self.prepare_firestore()
+
+    def prepare_firestore(self) -> None:
+        """Prepare the Firestore client.
+
+        Use this function to make sure your database is ready.
+        """
         self._document = self.firestore_client.collection(
             self.collection_name
         ).document(self.session_id)