[postgres] classify slot read failing due to inactive sync standby (#3523)

Co-authored-by: Kunal Gupta <[email protected]>

Author: heavycrystal

flow/alerting/classifier.go

@@ -437,6 +437,11 @@ func GetErrorClass(ctx context.Context, err error) (ErrorClass, ErrorInfo) {
 				return ErrorNotifySlotInvalid, pgErrorInfo
 			}
 
+			if strings.Contains(pgErr.Message,
+				`specified in parameter "synchronized_standby_slots" does not have active_pid`) {
+				return ErrorRetryRecoverable, pgErrorInfo
+			}
+
 		case pgerrcode.InvalidParameterValue:
 			if strings.Contains(pgErr.Message, "invalid snapshot identifier") {
 				return ErrorNotifyInvalidSnapshotIdentifier, pgErrorInfo