Add stats for part counts and sizes
Add stats for part counts and sizes

This adds states for number of parts and their sizes per table:

```
clickhouse_clickhouse_table_parts_bytes{database="r0",table="requests_internal_replica"} 5252
clickhouse_clickhouse_table_parts_bytes{database="r1",table="requests_internal_replica"} 5238
clickhouse_clickhouse_table_parts_bytes{database="r2",table="requests_internal_replica"} 5366
clickhouse_clickhouse_table_parts_bytes{database="system",table=".inner.clickhouse_query_stats_by_user"} 3061
clickhouse_clickhouse_table_parts_bytes{database="system",table="query_log"} 70451
clickhouse_clickhouse_table_parts_count{database="r0",table="requests_internal_replica"} 1
clickhouse_clickhouse_table_parts_count{database="r1",table="requests_internal_replica"} 1
clickhouse_clickhouse_table_parts_count{database="r2",table="requests_internal_replica"} 1
clickhouse_clickhouse_table_parts_count{database="system",table=".inner.clickhouse_query_stats_by_user"} 3
clickhouse_clickhouse_table_parts_count{database="system",table="query_log"} 4
```

f1yegor/clickhouse_exporter