[chore](backup) limit the involved tablets in a backup job #39987 (#40080)

cherry pick from #39987

fe/fe-common/src/main/java/org/apache/doris/common/Config.java

@@ -1526,6 +1526,15 @@ public class Config extends ConfigBase {
     @ConfField(mutable = true, masterOnly = true)
     public static int max_backup_restore_job_num_per_db = 10;
 
+    /**
+     * Control the max num of tablets per backup job involved.
+     */
+    @ConfField(mutable = true, masterOnly = true, description = {
+        "用于控制每次 backup job 允许备份的 tablet 上限，以避免 OOM",
+        "Control the max num of tablets per backup job involved, to avoid OOM"
+    })
+    public static int max_backup_tablets_per_job = 300000;
+
     /**
      * whether to ignore table that not support type when backup, and not report exception.
      */