HTTP Connection Manager: Add the ability to have per worker stats

HTTP Connection Manager: Add the ability to have per worker stats

*Description*:
> Add stats that will allow us to see imbalance among workers. This is particularly useful for mesh scenarios wherein you might use an [exact balance](https://www.envoyproxy.io/docs/envoy/latest/api-v3/config/listener/v3/listener.proto#config-listener-v3-listener-connectionbalanceconfig) to balance the connection but can still end up with imbalance at the request level.

[optional *Relevant Links*:]
>Any extra documentation required to understand the issue.