This match expression takes a floating point number between 0 and 1 (both inclusive) with 0 matching nothing and 1 matching all. It desugars to {$expr: {$rand: ...}}

Unlike $sample, which is an experession, $sampleRate is a match-expression, which can go inside .find() or {$match: _}.

For example, the pipeline:

{$match: {$sampleRate: 0.25}}

Would select on average 1/4 of the input documents. It could desugar to

{$match: {$expr: {$lt: [ {$rand: {}}, 0.25 ]}}}