Stable sort the stats

# Stable sort the stats

Using a map instead of a list means the output order for counters is no longer guaranteed; this sorts the keys alphabetically and produces stable output.

```
$ go run stable.go
    METRIC                VALUE
     alpha                  100
      beta                    2
   charlie                    3
$ go run stable.go
    METRIC                VALUE
     alpha                  100
      beta                    2
   charlie                    3
$ go run stable.go
    METRIC                VALUE
     alpha                  100
      beta                    2
   charlie                    3
```