*: fix tidb_server_conn_idle_duration_seconds prometheus expression in grafana json file (#21396)
*: fix tidb_server_conn_idle_duration_seconds prometheus expression in grafana json file (#21396)

Signed-off-by: crazycs520 <174f850cd6ed76c2653f481397d398dd570f1f87@gmail.com>
kyle-wang/tidb,pingcap/tidb,kyle-wang/tidb,ngaut/tidb,pingcap/tidb,pingcap/tidb,c4pt0r/tidb,c4pt0r/tidb,ngaut/tidb,kyle-wang/tidb,ngaut/tidb,c4pt0r/tidb,ngaut/tidb,ngaut/tidb,pingcap/tidb,coocood/tidb,pingcap/tidb,ngaut/tidb,c4pt0r/tidb,coocood/tidb,kyle-wang/tidb,c4pt0r/tidb,c4pt0r/tidb,kyle-wang/tidb,pingcap/tidb,kyle-wang/tidb