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.

this seems like it would only keep the last aggregated value, so calling it a "sample" may be a bit misleading, since it's not random.

I can see how there might be a use case to have an aggregator that would keep only the first, last, or a random value in each bucket, so maybe we can flesh this out to offer all three options? This may be useful for metrics where we don't necessarily care about previous values, but only about the most recent one.