available_in_bytes stat for persisted queue / rename free_space_in_bytes

- Version: 5.4.0
- Operating System: Linux
- Config File: queue.type set to persisted
- Steps to Reproduce:

Amount of free space reported in `queue.data.free_space_in_bytes` from `/_node/stats/pipeline` stats API doesn't take reserved space into account. Logstash is usually running under specific user that will not have rights to use all of this space.

It looks like Elasticsearch has an additional stat called `available_in_bytes` which provides a value that reflects reserved % properly. Could something similar be done in Logstash?

Also is it possible to rename `free_space_in_bytes` to `free_in_bytes` to be in sync with Elasticsearch convention?