Expose KeepInstanceInTheRingOnShutdown config for store gateway

[justinjung04]

What this PR does:

Add -store-gateway.sharding-ring.keep-instance-in-the-ring-on-shutdown.

The new config sets KeepInstanceInTheRingOnShutdown configuration in basic_lifecycler, which skips unregistering instance if set to true.

cortex/pkg/ring/basic_lifecycler.go

Lines 244 to 252 in 6dd44ee

	if l.cfg.KeepInstanceInTheRingOnShutdown {
	level.Info(l.logger).Log("msg", "keeping instance the ring", "ring", l.ringName)
	} else {
	// Remove the instance from the ring.
	if err := l.unregisterInstance(context.Background()); err != nil {
	return errors.Wrapf(err, "failed to unregister instance from the ring (ring: %s)", l.ringName)
	}
	level.Info(l.logger).Log("msg", "instance removed from the ring", "ring", l.ringName)
	}

Similar to -ingester.unregister-on-shutdown, this new config is helpful when you want to eliminate ring-sync during rollout deployment.

Which issue(s) this PR fixes:
n/a

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[yeya24]

Thanks!