tiflash: add two more TiFlash config items (#20050) (#20055)

tiflash/tiflash-configuration.md

@@ -183,6 +183,13 @@ tmp_path = "/tidb-data/tiflash-9000/tmp"
 
     ## New in v5.0. This item specifies the maximum number of cop requests that TiFlash Coprocessor executes at the same time. If the number of requests exceeds the specified value, the exceeded requests will queue. If the configuration value is set to 0 or not set, the default value is used, which is twice the number of physical cores.
     cop_pool_size = 0
+
+    ## New in v5.0. This item specifies the maximum number of cop requests that TiFlash Coprocessor handles at the same time, including the requests being executed and the requests waiting in the queue. If the number of requests exceeds the specified value, the error "TiFlash Server is Busy" is returned. -1 indicates no limit; 0 indicates using the default value, which is 10 * cop_pool_size.
+    cop_pool_handle_limit = 0
+
+    ## New in v5.0. This item specifies the maximum time that a cop request can queue in TiFlash. If a cop request waits in the queue for a time longer than the value specified by this configuration, the error "TiFlash Server is Busy" is returned. A value less than or equal to 0 indicates no limit.
+    cop_pool_max_queued_seconds = 15
+
     ## New in v5.0. This item specifies the maximum number of batch requests that TiFlash Coprocessor executes at the same time. If the number of requests exceeds the specified value, the exceeded requests will queue. If the configuration value is set to 0 or not set, the default value is used, which is twice the number of physical cores.
     batch_cop_pool_size = 0
     ## New in v5.4.0. This item enables or disables the elastic thread pool feature, which significantly improves CPU utilization in high concurrency scenarios of TiFlash. The default value is false. The elastic thread pool feature is experimental and not recommended for production environments.