GovukAppConfig silences OpenTelemetry log output when running a rake task

[KludgeKML]

#307 removes OpenTelemetry initialisation in the console to avoid flooding the console with OpenTelemetry startup messages.

This PR adds the additional behaviour that when OpenTelemetry is configured (ie we are not on a console), we do a second check to see whether we're in a rake task and silence OpenTelemetry log messages so that Rake tasks with simple text output are not swamped with OpenTelemetry startup messages.

Since we're already assuming Rails.const_defined? is available, we use it to check for Rake, then call Rake.application.top_level tasks, which returns an empty array if running in rails, but a list of tasks if running through rake.

[theseanything]

@KludgeKML ah sorry this had been a problem. So the telemetry can actually be usefully for Rake tasks, we can follow the requests paths of a rake task etc...

Happy to switch it off for now - but wondering if there's a nicer way just to silence this output.

[KludgeKML]

@theseanything I don't know much about how OpenTelemetry is configured, do you know if there is a way to silence the startup output?

[robinjam]

It looks like we could configure it not to log anything (perhaps only when invoked via a Rake task?)

OpenTelemetry::SDK.configure do |c|
  c.logger = Logger.new(File::NULL)
end

https://www.rubydoc.info/gems/opentelemetry-sdk/OpenTelemetry/SDK#configure-instance_method

[KludgeKML]

@robinjam / @theseanything I've updated this PR to try James's suggestion, will try it on a branch of email-alert-api to confirm it works as expected, then if that goes okay I'll tag you both in as reviewers.

[KludgeKML]

@theseanything Since you approved I've moved the changed code into lib/govuk_app_config/govuk_open_telemetry.rb instead of it being in the railtie (so that the OpenTelemetry::SDK config code is all in one block). That still okay?