For ListenerCounter implemented MarshalJSON() method.

internal/radio/listeners.go

@@ -1,6 +1,7 @@
 package radio
 
 import (
+	"encoding/json"
 	"errors"
 	"fmt"
 	"math"
@@ -58,3 +59,14 @@ func (l *ListenerCounter) Dec() (int, error) {
 	l.current--
 	return l.current, nil
 }
+
+func (l *ListenerCounter) MarshalJSON() ([]byte, error) {
+	l.mut.Lock()
+	defer l.mut.Unlock()
+	return json.Marshal(&struct {
+		Current int `json:"current"`
+		Peak    int `json:"peak"`
+	}{
+		Current: l.current,
+		Peak:    l.peak})
+}