-----Description-----  
This task requires writing a Lean 4 method that counts the number of digit characters within a given string. A digit is any character between '0' and '9'. The method should determine how many such digit characters appear in the input.

-----Input-----  
The input consists of:  
s: A string.

-----Output-----  
The output is a natural number (Nat):  
Returns a non-negative count representing the number of digit characters found in the input string.

-----Note-----  
There are no additional preconditions; the method works for any provided string.