Unexpected metric names when cluster name contains dot

*Title*: *Unexpected metric names when cluster name contains dot*

*Description*:
> When cluster name contains dot, cluster-specific metric names are unexpected.

*Repro steps*:
> Create a cluster with dots in its name, e.g. `foo.example.com`, and then look for cluster metrics like `envoy.cluster.upstream_rq_xx`

*Expected*:
> The metric should be named `envoy.cluster.upstream_rq_xx` with tag `envoy_cluster_name=foo.example.com`

*Actual*:
> The metric is named `envoy.cluster.example.com.upstream_rq_xx` with tag `envoy_cluster_name=foo`