Load queries for dashboard page from new system.dashboards table

docs/en/operations/system-tables/dashboards.md

@@ -0,0 +1,68 @@
+---
+slug: /en/operations/system-tables/dashboards
+---
+# dashboards
+
+Contains queries used by `/dashboard` page accessible though [HTTP interface](/docs/en/interfaces/http.md).
+This table can be useful for monitoring and troubleshooting. The table contains a row for every chart in a dashboard.
+
+:::note
+`/dashboard` page can render queries not only from `system.dashboards`, but from any table with the same schema.
+This can be useful to create custom dashboards.
+:::
+
+Example:
+
+``` sql
+SELECT *
+FROM system.dashboards
+WHERE title ILIKE '%CPU%'
+```
+
+``` text
+Row 1:
+──────
+dashboard: overview
+title:     CPU Usage (cores)
+query:     SELECT toStartOfInterval(event_time, INTERVAL {rounding:UInt32} SECOND)::INT AS t, avg(ProfileEvent_OSCPUVirtualTimeMicroseconds) / 1000000
+FROM system.metric_log
+WHERE event_date >= toDate(now() - {seconds:UInt32}) AND event_time >= now() - {seconds:UInt32}
+GROUP BY t
+ORDER BY t WITH FILL STEP {rounding:UInt32}
+
+Row 2:
+──────
+dashboard: overview
+title:     CPU Wait
+query:     SELECT toStartOfInterval(event_time, INTERVAL {rounding:UInt32} SECOND)::INT AS t, avg(ProfileEvent_OSCPUWaitMicroseconds) / 1000000
+FROM system.metric_log
+WHERE event_date >= toDate(now() - {seconds:UInt32}) AND event_time >= now() - {seconds:UInt32}
+GROUP BY t
+ORDER BY t WITH FILL STEP {rounding:UInt32}
+
+Row 3:
+──────
+dashboard: overview
+title:     OS CPU Usage (Userspace)
+query:     SELECT toStartOfInterval(event_time, INTERVAL {rounding:UInt32} SECOND)::INT AS t, avg(value)
+FROM system.asynchronous_metric_log
+WHERE event_date >= toDate(now() - {seconds:UInt32}) AND event_time >= now() - {seconds:UInt32} AND metric = 'OSUserTimeNormalized'
+GROUP BY t
+ORDER BY t WITH FILL STEP {rounding:UInt32}
+
+Row 4:
+──────
+dashboard: overview
+title:     OS CPU Usage (Kernel)
+query:     SELECT toStartOfInterval(event_time, INTERVAL {rounding:UInt32} SECOND)::INT AS t, avg(value)
+FROM system.asynchronous_metric_log
+WHERE event_date >= toDate(now() - {seconds:UInt32}) AND event_time >= now() - {seconds:UInt32} AND metric = 'OSSystemTimeNormalized'
+GROUP BY t
+ORDER BY t WITH FILL STEP {rounding:UInt32}
+```
+
+Columns:
+
+- `dashboard` (`String`) - The dashboard name.
+- `title` (`String`) - The title of a chart.
+- `query` (`String`) - The query to obtain data to be displayed.