The _total suffix is by convention for counters,
don't use it on a gauge. Clarify help string.
Tweak metric name so it'll sort with related metrics,
and be a little more understandable.
Remove open file descriptor metric, as Prometheus client_golang
provides that out of the box as process_open_fds which is also
more up to date. Both only support Linux, so there's no loss of
platform support.
Fixes#5229
Fix#2841.
From Prometheus developer:
```
the recommended way for etcd as an open source project and under
consideration of its size would be etcd_<subsystem>_<name>.
```
We made the naming change accordingly.
It exposes the metrics of file descriptor limit and file descriptor used.
Moreover, it prints out warning when more than 80% of fd limit has been used.
```
2015/04/08 01:26:19 etcdserver: 80% of the file descriptor limit is open
[open = 969, limit = 1024]
```
Xiang Li