Add script for ZFS snapshots

[lstrojny]

Generates two metrics for ZFS snapshots:

• zfs_snapshot_space_usage_bytes: snapshot space usage in bytes
• zfs_snapshot_latest_snapshot_taken: UNIX timestamp

[dswarbrick]

Please use the official Prometheus client_python library, rather than a homegrown metric printing function.

[lstrojny]

@dswarbrick done

[dswarbrick]

@lstrojny Can you also include some sample command outputs (in the mock/fixtures directory) and optionally a mock zfs script (see existing examples in mock directory, so that we can (eventually) write tests for this collector?

[dswarbrick]

Great stuff, thanks for your contribution.

PS: Any interest in merging the functionality of zfs_zpool.sh into this collector? :wink:

[lstrojny]

Thanks! I probably won’t find the time to move zfs_zpool.sh into this collector, although a general purpose zfs.py would be nice