optional string key = 1;

  // Represents a key's relationship to a set of values.
  // Valid operators are In, NotIn, Exists, DoesNotExist. Gt, and Lt.
  optional string operator = 2;

  // An array of string values. If the operator is In or NotIn,
  // the values array must be non-empty. If the operator is Exists or DoesNotExist,