Add metric for number of reserved replicas in a fleet

[roberthbailey]

Is your feature request related to a problem? Please describe.
The current metrics for fleets include total game servers, allocated game servers, ready game servers, and desired game servers. Since the reserved game server state was added in #766 we haven't updated the metrics to count game servers in this state.

Describe the solution you'd like
Update the recordFleetReplicas function to include reserved game servers:

agones/pkg/metrics/controller.go

Lines 256 to 268 in 418aff0

	func (c *Controller) recordFleetReplicas(fleetName, fleetNamespace string, total, allocated, ready, desired int32) {
	ctx, _ := tag.New(context.Background(), tag.Upsert(keyName, fleetName), tag.Upsert(keyNamespace, fleetNamespace))
	recordWithTags(ctx, []tag.Mutator{tag.Upsert(keyType, "total")},
	fleetsReplicasCountStats.M(int64(total)))
	recordWithTags(ctx, []tag.Mutator{tag.Upsert(keyType, "allocated")},
	fleetsReplicasCountStats.M(int64(allocated)))
	recordWithTags(ctx, []tag.Mutator{tag.Upsert(keyType, "ready")},
	fleetsReplicasCountStats.M(int64(ready)))
	recordWithTags(ctx, []tag.Mutator{tag.Upsert(keyType, "desired")},
	fleetsReplicasCountStats.M(int64(desired)))
	}

Describe alternatives you've considered
None. We should totally do this.

Additional context
n/a

[govargo]

I'd like to try this issue.
I'll send the PR fot this.

[roberthbailey]

Awesome!