Enable watchdog module by default; Add no data error (prometheus#2953)

Signed-off-by: Gavin Lam <gavin.oss@tutamail.com>

README.md

@@ -158,6 +158,7 @@ timex | Exposes selected adjtimex(2) system call stats. | Linux
 udp_queues | Exposes UDP total lengths of the rx_queue and tx_queue from `/proc/net/udp` and `/proc/net/udp6`. | Linux
 uname | Exposes system information as provided by the uname system call. | Darwin, FreeBSD, Linux, OpenBSD
 vmstat | Exposes statistics from `/proc/vmstat`. | Linux
+watchdog | Exposes statistics from `/sys/class/watchdog` | Linux
 xfs | Exposes XFS runtime statistics. | Linux (kernel 4.4+)
 zfs | Exposes [ZFS](http://open-zfs.org/) performance statistics. | FreeBSD, [Linux](http://zfsonlinux.org/), Solaris
 
@@ -204,7 +205,6 @@ softirqs | Exposes detailed softirq statistics from `/proc/softirqs`. | Linux
 sysctl | Expose sysctl values from `/proc/sys`. Use `--collector.sysctl.include(-info)` to configure. | Linux
 systemd | Exposes service and system status from [systemd](http://www.freedesktop.org/wiki/Software/systemd/). | Linux
 tcpstat | Exposes TCP connection status information from `/proc/net/tcp` and `/proc/net/tcp6`. (Warning: the current version has potential performance issues in high load situations.) | Linux
-watchdog | Exposes statistics from `/sys/class/watchdog` | Linux
 wifi | Exposes WiFi device and station statistics. | Linux
 xfrm | Exposes statistics from `/proc/net/xfrm_stat` | Linux
 zoneinfo | Exposes NUMA memory zone metrics. | Linux

collector/watchdog.go

@@ -17,9 +17,12 @@
 package collector
 
 import (
+	"errors"
 	"fmt"
+	"os"
 
 	"github.com/go-kit/log"
+	"github.com/go-kit/log/level"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/procfs/sysfs"
 )
@@ -30,7 +33,7 @@ type watchdogCollector struct {
 }
 
 func init() {
-	registerCollector("watchdog", defaultDisabled, NewWatchdogCollector)
+	registerCollector("watchdog", defaultEnabled, NewWatchdogCollector)
 }
 
 // NewWatchdogCollector returns a new Collector exposing watchdog stats.
@@ -99,6 +102,10 @@ func toLabelValue(ptr *string) string {
 func (c *watchdogCollector) Update(ch chan<- prometheus.Metric) error {
 	watchdogClass, err := c.fs.WatchdogClass()
 	if err != nil {
+		if errors.Is(err, os.ErrNotExist) || errors.Is(err, os.ErrPermission) || errors.Is(err, os.ErrInvalid) {
+			level.Debug(c.logger).Log("msg", "Could not read watchdog stats", "err", err)
+			return ErrNoData
+		}
 		return err
 	}