traces_per_query

key is statsReportKey (# operationName\nsignature) Note that the nested traces will not have a signature or details.operationName (because the key is adequate).

We also assume that traces don't have legacy_per_query_implicit_operation_name, and we don't require them to have details.raw_query (which would consume a lot of space and has privacy/data access issues, and isn't currently exposed by our app anyway).