Add a warning when node exporter runs as root

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

.promu.yml

@@ -7,7 +7,7 @@ repository:
 build:
     binaries:
         - name: node_exporter
-    flags: -a -tags 'netgo static_build'
+    flags: -a -tags 'netgo osusergo static_build'
     ldflags: |
         -X github.com/prometheus/common/version.Version={{.Version}}
         -X github.com/prometheus/common/version.Revision={{.Revision}}

node_exporter.go

@@ -18,6 +18,7 @@ import (
 	"net/http"
 	_ "net/http/pprof"
 	"os"
+	"os/user"
 	"sort"
 
 	"github.com/prometheus/common/promlog"
@@ -176,6 +177,9 @@ func main() {
 	}
 	level.Info(logger).Log("msg", "Starting node_exporter", "version", version.Info())
 	level.Info(logger).Log("msg", "Build context", "build_context", version.BuildContext())
+	if user, err := user.Current(); err == nil && user.Uid == "0" {
+		level.Warn(logger).Log("msg", "Node Exporter is running as root user. This exporter is designed to run as unpriviledged user, root is not required.")
+	}
 
 	http.Handle(*metricsPath, newHandler(!*disableExporterMetrics, *maxRequests, logger))
 	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {