feat: function exporter-toolkit/web/kingpinflag.AddFlags and common/promlog/flag.AddFlags support kingpin.CmdClause as argument

[fengxsong]

Currently those two functions only accept *kingpin.Application as the first argument, When we use these methods in subcommands, we must wrap them again instead of calling them directly. here's a proposal

type flagGroup interface {
	Flag(string, string) *kingpin.FlagClause
}

func AddFlags(a flagGroup, defaultAddress string) *web.FlagConfig ...

func AddFlags(a flagGroup, config *promlog.Config) ...

This change won't impact existing method calls and ensures compatibility.

[metalmatze]

It's probably easiest if you send a PR directly. Thanks!