"source" argument in constructor of Vectara (#11454)

Replace this entire comment with:
- **Description:** minor update to constructor to allow for
specification of "source"
  - **Tag maintainer:** @baskaryan
  - **Twitter handle:** @ofermend

libs/langchain/langchain/vectorstores/vectara.py

@@ -39,6 +39,7 @@ class Vectara(VectorStore):
         vectara_corpus_id: Optional[str] = None,
         vectara_api_key: Optional[str] = None,
         vectara_api_timeout: int = 60,
+        source: Optional[str] = "langchain",
     ):
         """Initialize with Vectara API."""
         self._vectara_customer_id = vectara_customer_id or os.environ.get(
@@ -59,6 +60,8 @@ class Vectara(VectorStore):
             )
         else:
             logger.debug(f"Using corpus id {self._vectara_corpus_id}")
+        self._source = source
+
         self._session = requests.Session()  # to reuse connections
         adapter = requests.adapters.HTTPAdapter(max_retries=3)
         self._session.mount("http://", adapter)
@@ -74,7 +77,7 @@ class Vectara(VectorStore):
             "x-api-key": self._vectara_api_key,
             "customer-id": self._vectara_customer_id,
             "Content-Type": "application/json",
-            "X-Source": "langchain",
+            "X-Source": self._source,
         }
 
     def _delete_doc(self, doc_id: str) -> bool: