Sample agg

This metric does a sampling, i.e. the first value to encounter is kept, the rest will just be collapsed into a single row.

Very useful for having a rough idea of what's going on in a super-high cardinality dimension w/o blowing space requirements.

Caveats: Missing specs, very grateful for a hit of how to best test this.

same here, use Short.MAX_VALUE  and explain +/- 1?