{
  $lookup: {
    from: <foreign collection>,
    localField: <field from local collection's documents>,
    foreignField: <field from foreign collection's documents>,
    let: { <var_1>: <expression>, …, <var_n>: <expression> },
    pipeline: [ <pipeline to run> ],
    as: <output array field>
  }
} 

Fields "from", "localField", "foreignField", and "as" represent collection/field names. They should not be abstracted during telemetry key gen. However they should be redacted/hashed by $telemetry. "let" should be treated as a normal let expression. And "pipeline" should be serialized/redacted recursively as a regular document source pipeline.