Add EXACT CARDINALITY commands

[e-dard]

Required for all non-trivial PRs

• Rebased/mergable
• Tests pass
• CHANGELOG.md updated

This PR introduces five new commands:

SHOW SERIES EXACT CARDINALITY [ON] [FROM] [WHERE] [GROUP BY] [LIMIT] [OFFSET]
SHOW MEASUREMENT EXACT CARDINALITY [ON] [FROM] [WHERE] [GROUP BY] [LIMIT] [OFFSET]
SHOW FIELD KEY EXACT CARDINALITY [ON] [FROM] [WHERE] [GROUP BY] [LIMIT] [OFFSET]
SHOW TAG KEY EXACT CARDINALITY [ON] [FROM] [WHERE] [GROUP BY] [LIMIT] [OFFSET]
SHOW TAG VALUES EXACT CARDINALITY [ON] [FROM] WITH KEY = "x" [WHERE] [GROUP BY] [LIMIT] [OFFSET]

Further, two versions of both the already-implemented SHOW SERIES CARDINALITY and SHOW MEASUREMENT CARDINALITY commands have been converted to utilise the HLL++ sketches that are maintained in the index. These estimations are very accurate, and very fast.

Specifically, the variants that currently support cardinality estimation are

SHOW SERIES CARDINALITY;
SHOW SERIES CARDINALITY ON db0;
SHOW MEASUREMENT CARDINALITY
SHOW MEASUREMENT CARDINALITY ON db0;

All other SHOW X CARDINALITY commands will continue to use the exact-counting approaches of the SHOW X EXACT CARDINALITY commands. They will be converted to estimations in future releases.

[e-dard]

@benbjohnson @jsternberg question over what your thoughts are on this name?

[benbjohnson]

I think cardinality works for me. 👍

[benbjohnson]

I think cardinality works for me. 👍

[jsternberg]

It seems to me like you should do this instead.

if s.Exact {
   _, _ = buf.WriteString(" EXACT")
}
_, _ = buf.WriteString(" CARDINALITY")

if s.Database != "" {
    // the stuff that's here
}

if !s.Exact {
    return buf.String()
}

Then there's less code repetition.

[jsternberg]

The same comment from above applies here.

[e-dard]

@jsternberg changes made. Thanks.