bugfix: Oban instance not available when named supervisors are used

[snackycracky]

Issue:

Oban is hardcoded, but the default instance is in some cases not available because a named supervisor is used.
see #260

Change description

What problem does this solve?

Exception when named functions are used:

[error] Error when calling MFA defined by measurement: PromEx.Plugins.Oban :execute_queue_metrics [MapSet.new([Oban, GridState.Jobs.Oban])]
Class=:error
Reason=%RuntimeError{
  message: "No Oban instance named `Oban` is running and config isn't available.\n"
}
Stacktrace=[
  {Oban.Registry, :config, 1, [file: ~c"lib/oban/registry.ex", line: 37]},
  {PromEx.Plugins.Oban, :include_zeros_for_missing_queue_states, 1,
   [file: ~c"lib/prom_ex/plugins/oban.ex", line: 452]},
  {PromEx.Plugins.Oban, :handle_oban_queue_polling_metrics, 2,
   [file: ~c"lib/prom_ex/plugins/oban.ex", line: 441]},
  {Enum, :"-each/2-fun-0-", 3, [file: ~c"lib/enum.ex", line: 992]},
  {Enum, :"-each/2-anonymous-3-", 3, [file: ~c"lib/enum.ex", line: 4511]},
  {Enumerable.List, :reduce, 3, [file: ~c"lib/enum.ex", line: 4964]},
  {Enum, :each, 2, [file: ~c"lib/enum.ex", line: 4511]},
  {:telemetry_poller, :make_measurement, 1,
   [
     file: ~c"/workdir/deps/telemetry_poller/src/telemetry_poller.erl",
     line: 471
   ]},
  {:telemetry_poller, :"-make_measurements_and_filter_misbehaving/1-lc$^0/1-0-",
   1,
   [
     file: ~c"/workdir/deps/telemetry_poller/src/telemetry_poller.erl",
     line: 467
   ]},
  {:telemetry_poller, :"-make_measurements_and_filter_misbehaving/1-lc$^0/1-0-",
   1,
   [
     file: ~c"/workdir/deps/telemetry_poller/src/telemetry_poller.erl",
     line: 467
   ]},
  {:telemetry_poller, :handle_info, 2,
   [
     file: ~c"/workdir/deps/telemetry_poller/src/telemetry_poller.erl",
     line: 492
   ]},
  {:gen_server, :try_handle_info, 3, [file: ~c"gen_server.erl", line: 2345]},
  {:gen_server, :handle_msg, 6, [file: ~c"gen_server.erl", line: 2433]},
  {:proc_lib, :init_p_do_apply, 3, [file: ~c"proc_lib.erl", line: 329]}
]

Example usage

see testcase

Additional details and screenshots

Checklist

• I have added unit tests to cover my changes.
• I have added documentation to cover my changes.
• My changes have passed unit tests and have been tested E2E in an example project.