[dbNode] Add filesystem available space reporting

[shrek]

Add a reporter to report on the available space in the filesystem path being used as the rootDir of the dbnode.

This can be used for alerting purposes.

The code change for this is here: https://github.com/shrek/m3/commit/2d1f72edb27ec206cc2d76a666367eaea340e40e

Per the contribution guidelines, I am opening this issue for discussion prior to opening a PR.

There is a grafana panel in Node Details dashboard already called "Disk Free Space" which is currently not implemented. That panel could be completed with these metrics.

[gibbscullen]

@shrek thanks for the contribution! We'll make sure to take a look / provide feedback shortly.

In the meantime, we would suggest also looking at alternative ways to track free space (such as node exporter).

[shrek]

@gibbscullen : thanks ! we are using the node-exporter, however, it would be nice to combine the reporting with m3db itself.

please take a look at this as well which I am adding in the same PR:

https://github.com/shrek/m3/commit/78648e85bc0931ce0597e3fc7791b2dffa256a08

to report on the sizes of directories used by m3db which goes towards addressing this: https://github.com/m3db/m3/issues/3244 filed by @asafm

ie, disk free reporting can be combined with directory usage reporting in the same PR.

Let me know any feedback on this.

[wesleyk]

@shrek thanks for the contribution! The approach seems to make sense, can you create a pull request and flesh out the test cases? Thank you!