br: add variable tidb_advancer_check_point_lag_limit to control log backup checkpoint lag (#21481)

Author: Leavrth

system-variables.md

@@ -1044,6 +1044,16 @@ mysql> SHOW GLOBAL VARIABLES LIKE 'max_prepared_stmt_count';
 - Unit: Bytes
 - This variable is used to control the threshold at which the TiDB server prefers to send read requests to a replica in the same availability zone as the TiDB server when [`tidb_replica_read`](#tidb_replica_read-new-in-v40) is set to `closest-adaptive`. If the estimated result is higher than or equal to this threshold, TiDB prefers to send read requests to a replica in the same availability zone. Otherwise, TiDB sends read requests to the leader replica.
 
+### tidb_advancer_check_point_lag_limit <span class="version-mark">New in v8.5.5 and v9.0.0</span>
+
+- Scope: GLOBAL
+- Persists to cluster: Yes
+- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
+- Type: Duration
+- Default value: `48h0m0s`
+- Range: `[1s, 8760h0m0s]`
+- This variable controls the maximum allowed checkpoint lag for a log backup task. If a task's checkpoint lag exceeds this limit, TiDB Advancer pauses the task.
+
 ### tidb_allow_tiflash_cop <span class="version-mark">New in v7.3.0</span>
 
 - Scope: SESSION | GLOBAL