Allow RF greater than number of zones to select more than one instance per zone

[justinjung04]

What this PR does:

ring.Get that should return n (or more) instances which form the replicas for the given key uses the following configs:

• ring.cfg.ReplicationFactor to control how many replicas you would like to use per key
• ring.cfg.ZoneAwarenessEnabled to allow replicas to be across the zones evenly

Problem is, when ZoneAwarenessEnabled is enabled and ReplicationFactor is greater than number of available zones, the function was silently ignoring the ReplicationFactor and returned only one instance from each zone:

cortex/pkg/ring/ring.go

Lines 376 to 380 in a307fc0

	if r.cfg.ZoneAwarenessEnabled && info.Zone != "" {
	if util.StringsContain(distinctZones, info.Zone) {
	continue
	}
	}

RF doesn't need to be bound to number of zones, and the following scenario is valid as well:

• There are 3 available zones
• Setting RF to 6 would choose 2 instances from each zone

This PR fixes the function to be able to return max ceil(RF/len(zones)) number of instances per zone. For instance, numOfZones = 3 and RF = 4 will return 4 instances, and each zone will have either 1 or 2 instances returned.

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]

[harry671003]

LGTM! Thanks Justin.

[yeya24]

Thanks, this looks good!