tidb-cloud: update metrics to add row-based standard storage

tidb-cloud/built-in-monitoring.md

@@ -107,7 +107,7 @@ The following table illustrates the instance-level main metrics under the **Inst
 | :------------| :------| :-------------------------------------------- |
 | Request Units | RU per second | The Request Unit (RU) is a unit of measurement used to track the resource consumption of a query or transaction in a {{{ .starter }}} instance. Besides user queries, background activities can also consume RUs, so when QPS is 0, RU usage per second might still be nonzero.|
 | Capacity vs Usage (RU/s) | Provisioned capacity (RCU), Consumed RU/s | The Request Capacity Units (RCUs) and the consumed Request Units (RU) per second in a {{{ .essential }}} instance. |
-| Used Storage Size | Row-based storage, Columnar storage | The size of row-based storage and columnar storage. This metric is displayed only when each storage type is 50 MiB or larger. |
+| Used Storage Size | Row-based storage, Row-based standard storage, Columnar storage | The size of row-based storage, row-based standard storage, and columnar storage. This metric is displayed only when the size of each storage type is 50 MiB or larger. Currently, **Row-based standard storage** has the same meaning as **Row-based storage**. |
 | Query Per Second | All, {SQL type} | The number of SQL statements executed per second, which are collected by SQL types, such as `SELECT`, `INSERT`, and `UPDATE`. |
 | Query Duration | Avg, P99, P99-{SQL type} | The duration from receiving a request from the client to the {{{ .starter }}} or {{{ .essential }}} instance until the instance executes the request and returns the result to the client. |
 | Failed Query | All | The number of SQL statement execution errors per second. |