Add some background update admin APIs (#11263)

Fixes #11259

docs/usage/administration/admin_api/background_updates.md

@@ -0,0 +1,84 @@
+# Background Updates API
+
+This API allows a server administrator to manage the background updates being
+run against the database.
+
+## Status
+
+This API gets the current status of the background updates.
+
+
+The API is:
+
+```
+GET /_synapse/admin/v1/background_updates/status
+```
+
+Returning:
+
+```json
+{
+    "enabled": true,
+    "current_updates": {
+        "<db_name>": {
+            "name": "<background_update_name>",
+            "total_item_count": 50,
+            "total_duration_ms": 10000.0,
+            "average_items_per_ms": 2.2,
+        },
+    }
+}
+```
+
+`enabled` whether the background updates are enabled or disabled.
+
+`db_name` the database name (usually Synapse is configured with a single database named 'master').
+
+For each update:
+
+`name` the name of the update.
+`total_item_count` total number of "items" processed (the meaning of 'items' depends on the update in question).
+`total_duration_ms` how long the background process has been running, not including time spent sleeping.
+`average_items_per_ms` how many items are processed per millisecond based on an exponential average.
+
+
+
+## Enabled
+
+This API allow pausing background updates.
+
+Background updates should *not* be paused for significant periods of time, as
+this can affect the performance of Synapse.
+
+*Note*: This won't persist over restarts.
+
+*Note*: This won't cancel any update query that is currently running. This is
+usually fine since most queries are short lived, except for `CREATE INDEX`
+background updates which won't be cancelled once started.
+
+
+The API is:
+
+```
+POST /_synapse/admin/v1/background_updates/enabled
+```
+
+with the following body:
+
+```json
+{
+    "enabled": false
+}
+```
+
+`enabled` sets whether the background updates are enabled or disabled.
+
+The API returns the `enabled` param.
+
+```json
+{
+    "enabled": false
+}
+```
+
+There is also a `GET` version which returns the `enabled` state.