code: @NonNull
      public static Expression avg(@NonNull Expression expression) {
          if (expression == null) {
              throw new IllegalArgumentException("expression cannot be null.");
          }
          return new Expression.FunctionExpression("AVG()", Arrays.asList(expression));
      }
docs: Creates an AVG(expr) function expression that returns the average of all the number values
  in the group of the values expressed by the given expression.
  
  @param expression The expression.
  @return The AVG(expr) function.