Add output of stderr in case of error to exec log message (#3862)

If the command failed with a non-zero exit status there might be an error
message on stderr. Append the first line to the error message to ease the
search for its cause.

(cherry picked from commit 8e515688eb)

plugins/inputs/exec/exec.go

@@ -41,6 +41,8 @@ const sampleConfig = `
   data_format = "influx"
 `
 
+const MaxStderrBytes = 512
+
 type Exec struct {
 	Commands []string
 	Command  string
@@ -96,15 +98,41 @@ func (c CommandRunner) Run(
 
 	cmd := exec.Command(split_cmd[0], split_cmd[1:]...)
 
-	var out bytes.Buffer
+	var (
+		out    bytes.Buffer
+		stderr bytes.Buffer
+	)
 	cmd.Stdout = &out
+	cmd.Stderr = &stderr
 
 	if err := internal.RunTimeout(cmd, e.Timeout.Duration); err != nil {
 		switch e.parser.(type) {
 		case *nagios.NagiosParser:
 			AddNagiosState(err, acc)
 		default:
-			return nil, fmt.Errorf("exec: %s for command '%s'", err, command)
+			var errMessage = ""
+			if stderr.Len() > 0 {
+				stderr = removeCarriageReturns(stderr)
+				// Limit the number of bytes.
+				didTruncate := false
+				if stderr.Len() > MaxStderrBytes {
+					stderr.Truncate(MaxStderrBytes)
+					didTruncate = true
+				}
+				if i := bytes.IndexByte(stderr.Bytes(), '\n'); i > 0 {
+					// Only show truncation if the newline wasn't the last character.
+					if i < stderr.Len()-1 {
+						didTruncate = true
+					}
+					stderr.Truncate(i)
+				}
+				if didTruncate {
+					stderr.WriteString("...")
+				}
+
+				errMessage = fmt.Sprintf(": %s", stderr.String())
+			}
+			return nil, fmt.Errorf("exec: %s for command '%s'%s", err, command, errMessage)
 		}
 	} else {
 		switch e.parser.(type) {