GCP Return credits field as json in bigquery. (#5100)

koku/masu/external/downloader/gcp/gcp_report_downloader.py

@@ -455,7 +455,7 @@ def build_query_select_statement(self):
         """Helper to build query select statement."""
         columns_list = self.gcp_big_query_columns.copy()
         columns_list = [
-            f"TO_JSON_STRING({col})" if col in ("labels", "system_labels", "project.labels") else col
+            f"TO_JSON_STRING({col})" if col in ("labels", "system_labels", "project.labels", "credits") else col
             for col in columns_list
         ]
         # Swap out resource columns with NULLs when we are processing