Support show tag values

[ShiKaiWi]

Describe This Problem

Currently, count(distinct(x)) is the only choice to fetch all the values of a tag, which requires scanning all the ssts and takes lots of time. However, this should be a very light-weight query for time series database, e.g. grafana shows suggestions about a specific label.

Proposal

• Record all the values for a tag if possible;
• Do fast query when count(distinct(x)) is performed on a tag column, that is to say, x column a tag column.

Additional Context

No response

[ShiKaiWi]

This is a huge task, I'll split it into these sub-tasks:

• [ ] Support separate index file for the column values
• [ ] Table options for supporting configure value index on specific column
• [ ] Query optimize rule for count(distinct(x)) where x has the value index
• [ ] Compaction support create index file for multiple ssts