Change client dialOpts append order

[rcanderson23]

Any dialOpts that are provided in the config but collide with what we set in dialGRPC() will be overridden. This change allows any config options to take precedent.

Example client snippet:

	var dialOpts []grpc.DialOption
	unary := mw.ChainUnaryClient(metadata.UnaryClientInterceptor, om.UnaryClientInterceptor(grpcMetrics))
	stream := mw.ChainStreamClient(metadata.StreamClientInterceptor, om.StreamClientInterceptor(grpcMetrics))

	dialOpts = append(dialOpts, grpc.WithUnaryInterceptor(unary), grpc.WithChainStreamInterceptor(stream))

	clt, err := client.New(ctx, client.Config{
		Addrs: []string{
			"teleport.carsonanderson.net:3025",
		},
		Credentials: []client.Credentials{
			client.LoadIdentityFile("teleport-auth.pem"),
		},
		DialOpts: dialOpts,
	})

Prior to this change, you wouldn't be able to setup a client and get grpc metrics with it (or any additional client interceptors).

The end goal is to get grpc client metrics on the proxy.

[rosstimothy]

LGTM

In regards to adding metrics, an interceptor may not necessarily be the best place to do so. Depending on what you are trying to capture a stats.Handler might be a better choice. See the work being done by @NajiObeid for proxy peering as an example: https://github.com/gravitational/teleport/pull/10440/files#diff-8d990324c72abe37d2e979e4f1d582d0de4429065256667df8ef597cc30fef55