[stats]: lazy init stats to save RAM (and CPU)

*Title*: stats: lazy init stats to save RAM (and CPU)

*Description*:

With lots of clusters and route-tables in a cloud proxy, we are seeing tons of RAM been spent on stats while most of the stats are never inc-ed due to traffic pattern(or long tail). We are thinking that we can lazy init cluster stats() so that the RAM is only allocated when it's required.

To achieve that we need to have finer grained stats group, e.g. configUpdateStats() are frequently updated by config management server, while upstream_xxx are only required when there is traffic for the cluster, for this sub-group we can save RAM by lazy init it.

Here is an example of cluster stats, when there is no traffic, Envoy is still paying the RAM cost to hold all the 0 stats, as well as the CPU that's burnt on collecting such stats.


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