fix: use oban name to retrieve config

[aleagnelli]

Change description

Retrieve Oban's config with the correct name

What problem does this solve?

Setting up an Oban instance with a custom name like this:

config :my_app, Oban,
  name: MyCustom.Oban,
  repo: MyApp.Repo,
  queues: [default: 10]

results in the following error

16:09:42.585 [error] Error when calling MFA defined by measurement: PromEx.Plugins.Oban :execute_queue_metrics [MapSet.new([MyCustom.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: 454]},
  {PromEx.Plugins.Oban, :handle_oban_queue_polling_metrics, 2,
   [file: ~c"lib/prom_ex/plugins/oban.ex", line: 443]},
  {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"/Users/ale/workspace/customoban/deps/telemetry_poller/src/telemetry_poller.erl",
     line: 336
   ]}
]

[aleagnelli]

Hi @akoutmos , just checking in on this PR whenever you get a chance.
Let me know if you need anything from me.

[snackycracky]

This solution has also a test: #271