fix: disk metrics error

[fuxingZhang]

fix issue #80

clickhouse_free_space_in_gigabytes and clickhouse_total_space_in_gigabytes are removed.
Because they can be calculated by free_space_in_bytes and total_space_in_bytes.

Visit http://localhost:9116/metrics, you can see:

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[fuxingZhang]

Thanks for the guidance, I have revised and resubmitted.