Update module github.com/prometheus/client_golang to v1.15.1

go.mod

@@ -6,6 +6,6 @@ require (
 	github.com/chrj/smtpd v0.3.1
 	github.com/jhillyerd/enmime v0.11.0
 	github.com/oschwald/geoip2-golang v1.8.0
-	github.com/prometheus/client_golang v1.11.0
+	github.com/prometheus/client_golang v1.15.1
 	golang.org/x/sync v0.1.0
 )