metricbeat: fix error when raid metrics are enabled on non-raid system

When the Linux Metrics integration (beta) is installed with the raid metrics
option enabled it causes an error if the host does not have a RAID
configuration. This error causes the Agent to go into a degraded state.

This happens because we report not having `/sys/block/md*` dirs  as an
error, this only means that no RAID configuration is in place.

To fix this, do not report this case as an error.

metricbeat/module/system/raid/blockinfo/getdev.go

@@ -44,7 +44,7 @@ func ListAll(path string) ([]MDDevice, error) {
 	}
 
 	if len(mds) == 0 {
-		return nil, fmt.Errorf("no matches from path %s", path)
+		return nil, nil
 	}
 
 	return mds, nil

metricbeat/module/system/raid/raid_test.go

@@ -18,6 +18,8 @@
 package raid
 
 import (
+	"os"
+	"path/filepath"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -46,6 +48,20 @@ func TestFetch(t *testing.T) {
 		events[0].BeatEvent("system", "raid").Fields.StringToPrint())
 }
 
+func TestFetchNoRAID(t *testing.T) {
+	// Ensure that we don't error out when no RAID devices are present
+	tmpDir := t.TempDir()
+	assert.NoError(t, os.MkdirAll(filepath.Join(tmpDir, "sys/block"), 0755))
+	c := getConfig()
+	c["hostfs"] = tmpDir
+
+	f := mbtest.NewReportingMetricSetV2Error(t, c)
+	events, errs := mbtest.ReportingFetchV2Error(f)
+
+	assert.Empty(t, errs)
+	assert.Empty(t, events)
+}
+
 func getConfig() map[string]interface{} {
 	return map[string]interface{}{
 		"module":     "system",