Fix deprecated gRPC naming.Watcher

[zendern]

Found as part of #1912 where we fixed most of the linting issues. The only outstanding item is as follows and has been ignored for now by adding a //nolint:staticcheck

pkg/querier/frontend/worker.go:60:10: SA1019: naming.Watcher is deprecated: please use package resolver.  (staticcheck)
	watcher naming.Watcher

cortex/pkg/querier/frontend/worker.go

Line 60 in 6774c62

watcher naming.Watcher

This will have ripple effects that will go up into where we define the resolver

cortex/pkg/querier/frontend/worker.go

Lines 76 to 96 in 6774c62

	resolver, err := naming.NewDNSResolverWithFreq(cfg.DNSLookupDuration)
	if err != nil {
	return nil, err
	}
	watcher, err := resolver.Resolve(cfg.Address)
	if err != nil {
	return nil, err
	}
	ctx, cancel := context.WithCancel(context.Background())
	w := &worker{
	cfg: cfg,
	log: log,
	server: server,
	ctx: ctx,
	cancel: cancel,
	watcher: watcher,
	}

There is no direct Watcher class that exists in the resolver package referenced. So the migration is likely a little more in depth than just using resolver.Watcher or something similar.

Reference to that Watcher class
https://godoc.org/google.golang.org/grpc/naming#Watcher

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[pracucci]

This is still valid

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[pracucci]

Still valid

[pracucci]

This will be required once we'll upgrade gRPC (1.30.0 or above), but we're currently blocked by etcd not supporting it yet. See this issue for more details: etcd-io/etcd#12124.

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[pracucci]

Still valid