Allow override for requesting specific worker types for Complement on command line. (#14324)

* Expose getting SYNAPSE_WORKER_TYPES from external, allowing override of workers requested.

* Add WORKER_TYPES variable option to complement.sh script that passes requested workers into start_for_complement.sh entrypoint.

* Update docs to reflect this new ability.

* Changelog

* Don't rely on soft wrapping to format long strings

Good idea dklimpel. Thanks for catching that.

Co-authored-by: Dirk Klimpel <5740567+dklimpel@users.noreply.github.com>

* Small nits just noticed in docs.

* Fixup new line in docs.

Co-authored-by: Dirk Klimpel <5740567+dklimpel@users.noreply.github.com>

docs/development/contributing_guide.md

@@ -324,6 +324,12 @@ The above will run a monolithic (single-process) Synapse with SQLite as the data
 
 - Passing `POSTGRES=1` as an environment variable to use the Postgres database instead.
 - Passing `WORKERS=1` as an environment variable to use a workerised setup instead. This option implies the use of Postgres.
+  - If setting `WORKERS=1`, optionally set `WORKER_TYPES=` to declare which worker
+    types you wish to test. A simple comma-delimited string containing the worker types
+    defined from the `WORKERS_CONFIG` template in
+    [here](https://github.com/matrix-org/synapse/blob/develop/docker/configure_workers_and_start.py#L54).
+    A safe example would be `WORKER_TYPES="federation_inbound, federation_sender, synchrotron"`.
+    See the [worker documentation](../workers.md) for additional information on workers.
 
 To increase the log level for the tests, set `SYNAPSE_TEST_LOG_LEVEL`, e.g:
 ```sh