Add a metric for amount of available space

It'd be convenient to expose a metric for how much space there is available on each tablet server (accounting for fs_wal_dir_reserved_bytes and fs_data_dirs_reserved_bytes). This would be useful in implementing a replica placement policy based on available space.

It's probably worth separating metrics for available WAL directory space and available data directory space. E.g. we may want to treat a lack of space differently depending on whether we are limited on WAL space and not limited on data space, and vice versa.