return error exit status in prometheus cli (#4296)

Signed-off-by: mikeykhalil <mikeyfkhalil@gmail.com>

cmd/prometheus/main.go

@@ -595,6 +595,7 @@ func main() {
 	}
 	if err := g.Run(); err != nil {
 		level.Error(logger).Log("err", err)
+		os.Exit(1)
 	}
 	level.Info(logger).Log("msg", "See you next time!")
 }

cmd/prometheus/main_test.go

@@ -20,6 +20,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"syscall"
 	"testing"
 	"time"
 
@@ -155,3 +156,20 @@ func TestComputeExternalURL(t *testing.T) {
 		}
 	}
 }
+
+// Let's provide an invalid configuration file and verify the exit status indicates the error.
+func TestFailedStartupExitCode(t *testing.T) {
+	fakeInputFile := "fake-input-file"
+	expectedExitStatus := 1
+
+	prom := exec.Command(promPath, "--config.file="+fakeInputFile)
+	err := prom.Run()
+	testutil.NotOk(t, err, "")
+
+	if exitError, ok := err.(*exec.ExitError); ok {
+		status := exitError.Sys().(syscall.WaitStatus)
+		testutil.Equals(t, expectedExitStatus, status.ExitStatus())
+	} else {
+		t.Errorf("unable to retrieve the exit status for prometheus: %v", err)
+	}
+}