The option `services.prometheus.environmentFile` has been removed since it was causing [issues](https://github.com/NixOS/nixpkgs/issues/126083) and Prometheus now has native support for secret files.
The new option `services.prometheus.enableReload` has been introduced
which, when enabled, causes the prometheus systemd service to reload
when its config file changes.
More specifically the following property holds: switching to a
configuration (`switch-to-configuration`) that changes the prometheus
configuration only finishes successully when prometheus has finished
loading the new configuration.
`enableReload` is `false` by default in which case the old semantics
of restarting the prometheus systemd service are in effect.
Upstream has apparently changed the configuration format and is now
throwing an error when the `encrypt_sse` option is set. According to the
current version of the documentation encryption moved to the
`sse_config` option that (is optional and) offers all the features we do
not use or care about for this test.
This commit adds an assertion that checks that either `configFile` or
`configuration` is configured for alertmanager. The alertmanager config
can not be an empty attributeset. The check executed with `amtool` fails
before the service even has the chance to start. We should probably not
allow a broken alertmanager configuration anyway.
This also introduces a test for alertmanager configuration that piggy
backs on the existing prometheus tests.
