Add an admin API for users' media statistics (#8700)

Add `GET /_synapse/admin/v1/statistics/users/media` to get statisics about local media usage by users.
Related to #6094
It is the first API for statistics.
Goal is to avoid/reduce usage of sql queries like [Wiki analyzing Synapse](https://github.com/matrix-org/synapse/wiki/SQL-for-analyzing-Synapse-PostgreSQL-database-stats)

Signed-off-by: Dirk Klimpel dirk@klimpel.org

docs/admin_api/statistics.md

@@ -0,0 +1,83 @@
+# Users' media usage statistics
+
+Returns information about all local media usage of users. Gives the
+possibility to filter them by time and user.
+
+The API is:
+
+```
+GET /_synapse/admin/v1/statistics/users/media
+```
+
+To use it, you will need to authenticate by providing an `access_token`
+for a server admin: see [README.rst](README.rst).
+
+A response body like the following is returned:
+
+```json
+{
+  "users": [
+    {
+      "displayname": "foo_user_0",
+      "media_count": 2,
+      "media_length": 134,
+      "user_id": "@foo_user_0:test"
+    },
+    {
+      "displayname": "foo_user_1",
+      "media_count": 2,
+      "media_length": 134,
+      "user_id": "@foo_user_1:test"
+    }
+  ],
+  "next_token": 3,
+  "total": 10
+}
+```
+
+To paginate, check for `next_token` and if present, call the endpoint
+again with `from` set to the value of `next_token`. This will return a new page.
+
+If the endpoint does not return a `next_token` then there are no more
+reports to paginate through.
+
+**Parameters**
+
+The following parameters should be set in the URL:
+
+* `limit`: string representing a positive integer - Is optional but is
+  used for pagination, denoting the maximum number of items to return
+  in this call. Defaults to `100`.
+* `from`: string representing a positive integer - Is optional but used for pagination,
+  denoting the offset in the returned results. This should be treated as an opaque value
+  and not explicitly set to anything other than the return value of `next_token` from a
+  previous call. Defaults to `0`.
+* `order_by` - string - The method in which to sort the returned list of users. Valid values are:
+  - `user_id` - Users are ordered alphabetically by `user_id`. This is the default.
+  - `displayname` - Users are ordered alphabetically by `displayname`.
+  - `media_length` - Users are ordered by the total size of uploaded media in bytes.
+    Smallest to largest.
+  - `media_count` - Users are ordered by number of uploaded media. Smallest to largest.
+* `from_ts` - string representing a positive integer - Considers only
+  files created at this timestamp or later. Unix timestamp in ms.
+* `until_ts` - string representing a positive integer - Considers only
+  files created at this timestamp or earlier. Unix timestamp in ms.
+* `search_term` - string - Filter users by their user ID localpart **or** displayname.
+  The search term can be found in any part of the string.
+  Defaults to no filtering.
+* `dir` - string - Direction of order. Either `f` for forwards or `b` for backwards.
+  Setting this value to `b` will reverse the above sort order. Defaults to `f`.
+
+
+**Response**
+
+The following fields are returned in the JSON response body:
+
+* `users` - An array of objects, each containing information
+  about the user and their local media. Objects contain the following fields:
+  - `displayname` - string - Displayname of this user.
+  - `media_count` - integer - Number of uploaded media by this user.
+  - `media_length` - integer - Size of uploaded media in bytes by this user.
+  - `user_id` - string - Fully-qualified user ID (ex. `@user:server.com`).
+* `next_token` - integer - Opaque value used for pagination. See above.
+* `total` - integer - Total number of users after filtering.