Add more aurora metrics to export

[jbripley]

Add helper method to create metric description to cut down on boilerplate.

[StephanErb]

There are a few metrics which seem to be in the wrong category. To prevent merge conflicts it would probably be best if you change them in this pull request.

Should be counter:

• jvm_class_loaded_count
• jvm_class_unloaded_count
• jvm_memory_non_heap_mb_max

Should be gauge:

• schedule_queue_size
• pubsub_executor_queue_size

[tommyulfsparre]

@jbripley Beautiful, thx!