Count Digit

Input: a := 1234
Output: 4

Input: a := 98
Output: 2

Input: a := 5
Output: 1

where a > 0

Using Loop

package main

import "fmt"

func main() {
    d := digitCount(99999)
    fmt.Println(d)
}

func digitCount(n int) int {
    result := 0
    for n > 0 {
        result++
        n = n / 10
    }
    return result
}

Dry Run
d = 123
Initially result = 0
After 1st Iteration: x = 12 and res=1
After 2nd Iteration: x = 1 and res=2
After 3rd Iteration: x = 0 and res=3

Time Complexity: θ(d) where d is number of digits

Using Logarithm

package main

import (
    "fmt"
    "math"
)

func main() {
    n := 99999
    d := int(math.Log10(float64(n))) + 1
    fmt.Println(d)
}