Enable NFS client metrics by default. (#828)

Enable NFS client metrics by default now that it nolonger prints errors
on scrape if there are no metrics to display.

Also fixup the nfsd README to match the nfs entry.

README.md

@@ -41,7 +41,8 @@ mdadm | Exposes statistics about devices in `/proc/mdstat` (does nothing if no `
 meminfo | Exposes memory statistics. | Darwin, Dragonfly, FreeBSD, Linux, OpenBSD
 netdev | Exposes network interface statistics such as bytes transferred. | Darwin, Dragonfly, FreeBSD, Linux, OpenBSD
 netstat | Exposes network statistics from `/proc/net/netstat`. This is the same information as `netstat -s`. | Linux
-nfsd | Exposes NFS kernel server statistics from `/proc/net/rpc/nfsd`. | Linux
+nfs | Exposes NFS client statistics from `/proc/net/rpc/nfs`. This is the same information as `nfsstat -c`. | Linux
+nfsd | Exposes NFS kernel server statistics from `/proc/net/rpc/nfsd`. This is the same information as `nfsstat -s`. | Linux
 sockstat | Exposes various statistics from `/proc/net/sockstat`. | Linux
 stat | Exposes various statistics from `/proc/stat`. This includes boot time, forks and interrupts. | Linux
 textfile | Exposes statistics read from local disk. The `--collector.textfile.directory` flag must be set. | _any_
@@ -66,7 +67,6 @@ ksmd | Exposes kernel and system statistics from `/sys/kernel/mm/ksm`. | Linux
 logind | Exposes session counts from [logind](http://www.freedesktop.org/wiki/Software/systemd/logind/). | Linux
 meminfo\_numa | Exposes memory statistics from `/proc/meminfo_numa`. | Linux
 mountstats | Exposes filesystem statistics from `/proc/self/mountstats`. Exposes detailed NFS client statistics. | Linux
-nfs | Exposes NFS client statistics from `/proc/net/rpc/nfs`. This is the same information as `nfsstat -c`. | Linux
 ntp | Exposes local NTP daemon health to check [time](./docs/TIME.md) | _any_
 qdisc | Exposes [queuing discipline](https://en.wikipedia.org/wiki/Network_scheduler#Linux_kernel) statistics | Linux
 runit | Exposes service status from [runit](http://smarden.org/runit/). | _any_

collector/nfs_linux.go

@@ -40,7 +40,7 @@ type nfsCollector struct {
 }
 
 func init() {
-	registerCollector("nfs", defaultDisabled, NewNfsCollector)
+	registerCollector("nfs", defaultEnabled, NewNfsCollector)
 }
 
 // NewNfsCollector returns a new Collector exposing NFS statistics.