Make background updates controllable via a plugin (#11306)

Co-authored-by: Brendan Abolivier <babolivier@matrix.org>

docs/modules/background_update_controller_callbacks.md

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+# Background update controller callbacks
+
+Background update controller callbacks allow module developers to control (e.g. rate-limit)
+how database background updates are run. A database background update is an operation
+Synapse runs on its database in the background after it starts. It's usually used to run
+database operations that would take too long if they were run at the same time as schema
+updates (which are run on startup) and delay Synapse's startup too much: populating a
+table with a big amount of data, adding an index on a big table, deleting superfluous data,
+etc.
+
+Background update controller callbacks can be registered using the module API's
+`register_background_update_controller_callbacks` method. Only the first module (in order
+of appearance in Synapse's configuration file) calling this method can register background
+update controller callbacks, subsequent calls are ignored.
+
+The available background update controller callbacks are:
+
+### `on_update`
+
+_First introduced in Synapse v1.49.0_
+
+```python
+def on_update(update_name: str, database_name: str, one_shot: bool) -> AsyncContextManager[int]
+```
+
+Called when about to do an iteration of a background update. The module is given the name
+of the update, the name of the database, and a flag to indicate whether the background
+update will happen in one go and may take a long time (e.g. creating indices). If this last
+argument is set to `False`, the update will be run in batches.
+
+The module must return an async context manager. It will be entered before Synapse runs a 
+background update; this should return the desired duration of the iteration, in
+milliseconds.
+
+The context manager will be exited when the iteration completes. Note that the duration
+returned by the context manager is a target, and an iteration may take substantially longer
+or shorter. If the `one_shot` flag is set to `True`, the duration returned is ignored.
+
+__Note__: Unlike most module callbacks in Synapse, this one is _synchronous_. This is
+because asynchronous operations are expected to be run by the async context manager.
+
+This callback is required when registering any other background update controller callback.
+
+### `default_batch_size`
+
+_First introduced in Synapse v1.49.0_
+
+```python
+async def default_batch_size(update_name: str, database_name: str) -> int
+```
+
+Called before the first iteration of a background update, with the name of the update and
+of the database. The module must return the number of elements to process in this first
+iteration.
+
+If this callback is not defined, Synapse will use a default value of 100.
+
+### `min_batch_size`
+
+_First introduced in Synapse v1.49.0_
+
+```python
+async def min_batch_size(update_name: str, database_name: str) -> int
+```
+
+Called before running a new batch for a background update, with the name of the update and
+of the database. The module must return an integer representing the minimum number of
+elements to process in this iteration. This number must be at least 1, and is used to
+ensure that progress is always made.
+
+If this callback is not defined, Synapse will use a default value of 100.