metadata run plugin metadata() order vary

[yifan513]

What happened:
metadata run plugin metadata() order vary.
For me I have two plugins have the func
Metadata(ctx context.Context, state request.Request)
it should be calling in
func (m *Metadata) ServeDNS...
in a fixed order.

But when I start server, it may runs inverse order as before.

What you expected to happen:
Run in a fixed order.

How to reproduce it (as minimally and precisely as possible):
If you setup 2 plugin has metadata, you can check

func setup(c *caddy.Controller) error {
	m, err := metadataParse(c)
	if err != nil {
		return err
	}
	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
		m.Next = next
		return m
	})

	c.OnStartup(func() error {
		plugins := dnsserver.GetConfig(c).Handlers()
		for _, p := range plugins {
			if met, ok := p.(Provider); ok {
				m.Providers = append(m.Providers, met)
			}
		}
		return nil
	})

	return nil
}

If you set a breakpoint to

plugins := dnsserver.GetConfig(c).Handlers()

you should see sometimes plugins items order changes.
This will cause order of items in m.Providers changes.
I added two images run runs without change corefile. You can see the redis and sortByLoc orders changes in the plugins.

Anything else we need to know?:

Environment:
Distributor ID: Ubuntu
Description: Ubuntu 20.04.6 LTS
Release: 20.04
Codename: focal

• the version of CoreDNS: 1.9.3

• Corefile:

• logs, if applicable:

• OS (e.g: cat /etc/os-release):
  
  

• Others: