Checkbox for enabling statistics of webservices allows to enter expression

When on configuration - subsystems - webservices, user can use Shift + click on checkbox to enter an expression for the value instead of just checking/unchecking. However, an attempt to enter an expression results in error because 'statistics-enabled' attribute of webservices subsystem doesn't allow an expression as its value.

"statistics-enabled" => {
    "type" => BOOLEAN,
    "description" => "Whether statistics are to be gathered for endpoints, default value is 'false'.",
    "expressions-allowed" => false,
    "nillable" => true,
    "default" => false,
    "access-type" => "read-write",
    "storage" => "configuration",
    "restart-required" => "no-services"
},

Web console could detect this and only allow Shift+click on boolean attributes that can use expressions.