Invalid metric format emitted for stats where to-zone/from-zone is empty

*Title*: *Invalid metric format emitted for stats where to-zone/from-zone is empty*

*Description*:
>What issue is being seen? Describe what should be happening instead of
the bug, for example: Envoy should not crash, the expected value isn't
returned, etc.

Stats are being emitted from Envoy with consecutive `..` between namespaces due to empty to-zone,
for example:
```
cluster.egress_dynamodb_iad.zone.1a..upstream_rq_2xx
```

*Repro steps*:
> Include sample requests, environment, etc. All data and inputs
required to reproduce the bug.

Do not specify an AZ for an upstream cluster.
Inspect the AZ-AZ stats via curl on admin.

*Admin and Stats Output*:
>Include the admin output for the following endpoints: /stats,
/clusters, /routes, /server_info. For more information, refer to the
[admin endpoint documentation.](https://www.envoyproxy.io/docs/envoy/latest/operations/admin)

```
> /stats
cluster.egress_dynamodb_iad.zone.1b..upstream_rq_200: 290642
cluster.egress_dynamodb_iad.zone.1b..upstream_rq_2xx: 290642
cluster.egress_dynamodb_iad.zone.1b..upstream_rq_completed: 290642
```