Change Firestore Metric Field Name to Load into Snowflake (#321)

### Description
- `response_received_at` is a nullable field. Specifically, if a request
errored out, then this field is null.
- Changing to put `sent_at` field into snowflake's `created_at` column
instead

### Tests
- Tested manually on dev server
<img width="680" alt="image"
src="https://github.com/astronomer/ask-astro/assets/26350341/00d19423-3947-4896-8f62-00ddd343dc55">
Verified in snowflake
<img width="1910" alt="image"
src="https://github.com/astronomer/ask-astro/assets/26350341/82b0199e-cfd0-4607-9642-bbf4dfd0bac5">

airflow/dags/metrcis/load_firestore_to_snowflake.py

@@ -49,8 +49,8 @@ def load_request_data_from_firestore() -> list[tuple[str, int, bool, int]]:
         end_ts = time.mktime(yesterday.timetuple())
 
         docs = (
-            requests_col.where(filter=FieldFilter("response_received_at", ">=", start_ts))
-            .where(filter=FieldFilter("response_received_at", "<", end_ts))
+            requests_col.where(filter=FieldFilter("sent_at", ">=", start_ts))
+            .where(filter=FieldFilter("sent_at", "<", end_ts))
             .stream()
         )
 
@@ -60,9 +60,9 @@ def load_request_data_from_firestore() -> list[tuple[str, int, bool, int]]:
             uuid = doc_dict["uuid"]
             score = doc_dict.get("score")
             status = doc_dict.get("status") == "complete"
-            response_received_at = datetime.fromtimestamp(doc_dict.get("response_received_at"))
+            sent_at = datetime.fromtimestamp(doc_dict.get("sent_at"))
             client = doc_dict.get("client")
-            rows.append((uuid, score, status, response_received_at, client))
+            rows.append((uuid, score, status, sent_at, client))
 
         return rows