NOOP `Filesystem` dimension
NOOP `Filesystem` dimension

This Dimension holds the block device for a mount point:
https://github.com/pebble/cloudwatch-mon-scripts-python/blob/master/cloudwatchmon/cli/put_instance_stats.py#L380

This is red herring information when we always run the monitoring script
in a Docker container.

We can't just _drop_ filesystem without affecting existing metrics, so
leave it "empty". (`-` is the value returned for the root partition,
`--disk-path=/`` in docker).

pebble/cloudwatch-mon-scripts-python,pebble/cloudwatch-mon-scripts-python