Empty collection should return success=0 (#1323, #1723)

Signed-off-by: Li Ling <liiling@google.com>

collector/pressure_linux.go

@@ -89,7 +89,7 @@ func (c *pressureStatsCollector) Update(ch chan<- prometheus.Metric) error {
 		vals, err := c.fs.PSIStatsForResource(res)
 		if err != nil {
 			level.Debug(c.logger).Log("msg", "pressure information is unavailable, you need a Linux kernel >= 4.20 and/or CONFIG_PSI enabled for your kernel")
-			return nil
+			return ErrNoData
 		}
 		switch res {
 		case "cpu":

collector/rapl_linux.go

@@ -50,7 +50,7 @@ func (c *raplCollector) Update(ch chan<- prometheus.Metric) error {
 	// nil zones are fine when platform doesn't have powercap files present.
 	zones, err := sysfs.GetRaplZones(c.fs)
 	if err != nil {
-		return nil
+		return ErrNoData
 	}
 
 	for _, rz := range zones {

collector/zfs.go

@@ -65,6 +65,14 @@ func NewZFSCollector(logger log.Logger) (Collector, error) {
 }
 
 func (c *zfsCollector) Update(ch chan<- prometheus.Metric) error {
+
+	if _, err := c.openProcFile(c.linuxProcpathBase); err != nil {
+		if err == errZFSNotAvailable {
+			level.Debug(c.logger).Log("err", err)
+			return ErrNoData
+		}
+	}
+
 	for subsystem := range c.linuxPathMap {
 		if err := c.updateZfsStats(subsystem, ch); err != nil {
 			if err == errZFSNotAvailable {