Provide total available system memory on the server

[spzala]

Currently we don't provide or display total available system memory on the server as part of the endpoint metrics. We need to find a good way to provide it similar to what prometheus provides process_resident_memory_bytes that we shows the memory consumption. For more discussion refer to the #9185

[spzala]

So it seems like to get host memory we need to add support for https://github.com/prometheus/node_exporter which might not be worth adding just for on thing. What we are showing today about process memory seems good enough and simple. Closing it after brief discussion with @gyuho Thanks!

[spzala]

If we hear any such requirement from etcd users in future, we will reopen the issue.