code: public static <TExpression> Bson computed(final String fieldName, final TExpression expression) {
          return new SimpleExpression<TExpression>(fieldName, expression);
      }
docs: Creates a projection of a field whose value is computed from the given expression.  Projection with an expression is only supported
  using the $project aggregation pipeline stage.
  
  @param fieldName     the field name
  @param expression    the expression
  @param <TExpression> the expression type
  @return the projection
  @see Aggregates#project(Bson)