- When using Active Record async feature, the instrumentation was
  not thread safe.

  ```ruby
  Post.count_async

  ActiveSupport::Notifications.subscribed(->(*) { }, "active_record.sql") do
    Post.count
  end
  ```

  With the right race condition, the subscription would never fire up.
  This is due to temporarly overriding the instrumentation instance
  variable on the connection.