Add server.live stat to indicate if Envoy Gateway xds server is live/not draining

*Description*:
Currently, the is no utility stat to indicate if xds server (or even entire Envoy Gateway which consists of multiple components) is live or not. One could rely on `process_cpu_seconds_total` but it would be convenient to have a dedicated metrics that also takes draining state into account.

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