stats for route/virtual host/host or any resource with metadata

Now Envoy provides users larger number of metrics of listener and upstream cluster. It helps a lot.

But we still want some more detailed request stats for route or host. For example, `5xx` number of `/route1`, `4xx` number of `1.2.3.4:80` or attach some more labels from control plane to stats.

Now we have `virtual cluster` which can provides some more stats. But It's hard to create a `virtual cluster` for every route and the `virtual cluster` also need some external match which will degrade performance.