*: optimize runtime.FDUsage + add OS level FD metrics

[gyuho]

No need to count when we just want the count.

And expose it via metrics that can be used for setting up the monitoring and alarms:

(Prometheus exposes similar ones, while os_fd_... are the ones perceived by etcd)

...
# HELP os_fd_limit The file descriptor limit.
# TYPE os_fd_limit gauge
os_fd_limit 120000
# HELP os_fd_used The number of used file descriptors.
# TYPE os_fd_used gauge
os_fd_used 14
# HELP process_cpu_seconds_total Total user and system CPU time spent in seconds.
# TYPE process_cpu_seconds_total counter
process_cpu_seconds_total 0.31
# HELP process_max_fds Maximum number of open file descriptors.
# TYPE process_max_fds gauge
process_max_fds 120000
# HELP process_open_fds Number of open file descriptors.
# TYPE process_open_fds gauge
process_open_fds 17
...

[gyuho]

/cc @xiang90 @cfc4n @tangcong @spzala

[tangcong]

lgtm. thanks.