Make oban metrics buckets configurable

[szajbus]

Change description

What problem does this solve?

Defaults for histogram buckets rarely fit every app. This change allows to configure them.

Example usage

def plugins do
  [
    ...
    {PromEx.Plugins.Oban, oban_supervisors: [Oban], job_duration_buckets: [10, 100, 500, 1_000, 5_000, 10_000, 20_000]}
  ]
end

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.