feat: #234 support kingpin.CmdClause as argument (#238)

Signed-off-by: fengxusong <[email protected]>

web/kingpinflag/flag.go

@@ -19,10 +19,16 @@ import (
 	"github.com/prometheus/exporter-toolkit/web"
 )
 
-// AddFlags adds the flags used by this package to the Kingpin application.
+type flagGroup interface {
+	Flag(string, string) *kingpin.FlagClause
+}
+
+var _ flagGroup = &kingpin.Application{}
+
+// AddFlags adds the flags used by this package to the Kingpin application or CmdClause.
 // To use the default Kingpin application, call
 // AddFlags(kingpin.CommandLine, ":portNum") where portNum is the default port.
-func AddFlags(a *kingpin.Application, defaultAddress string) *web.FlagConfig {
+func AddFlags(a flagGroup, defaultAddress string) *web.FlagConfig {
 	systemdSocket := func() *bool { b := false; return &b }() // Socket activation only available on Linux
 	if runtime.GOOS == "linux" {
 		systemdSocket = a.Flag(