Improve logging around 'queue is full' error

This was first considered with https://github.com/elastic/apm-server/issues/990. At the time, it was closed, but it might be worth revisiting now that some time has passed.

The 'queue is full' error can indicate a number of different issues. It would be very helpful if we could improve the logging around the 'queue is full' event to include some more information, such as what is being returned from Elasticsearch or Logstash, and what the current state of the queue itself is. It would also be helpful to allow an option to log the current state of the queue and how many events it contains, even if it is a debug level setting. Exposing it so metricbeat can collect queue statistics would be even better.