Add support for instance-level throttles

[tessereth]

I've been trying to set up throttles for my maintenance tasks based on configurable values and it's pretty tricky to do in the current code. For instance, I want to be able to configure a database load threshold and tweak the backoff using an attribute. I'm also running the same maintenance task multiple times against different databases and I need different throttle parameters for each. Currently, throttles are set at the class level, so they can't vary per instance.

This PR adds support for per-instance throttles, using basically the same API as the current class-level throttles. Usage looks like:

class MyMaintenanceTask < MaintenanceTasks::Task
  attribute :database, , inclusion: { in: DatabaseShards.all }
  attribute :throttle_db_load_limit, :integer, default: 16
  attribute :throttle_db_load_backoff, :integer, default: 3600

  def initialize(*)
    super

    throttle_on(backoff: throttle_db_load_backoff) do
      ApplicationRecord.connected_to(shard: database) do
        current_load >= throttle_db_load_limit
      end
    end
  end
end

The throttle conditions array is only accessed here via the instance, so I've just overridden this method:

maintenance_tasks/app/jobs/concerns/maintenance_tasks/task_job_concern.rb

Line 98 in 58e544a

@task.throttle_conditions.reduce(collection_enum) do |enum, condition|

Please let me know what you think. I'm happy to change things if you can think of a better way to achieve the outcomes I'm after.

[tessereth]

The CLA is now signed, I hope you get a chance to take a look at this.