Add special syslog timestamp parser that uses current year (#4190)

Previously it was impossible to parse syslog timestamps without the date
being reported as year 0, due to the year not being specified

plugins/inputs/logparser/grok/grok_test.go

@@ -970,3 +970,15 @@ func TestNewlineInPatterns(t *testing.T) {
 	require.NoError(t, err)
 	require.NotNil(t, m)
 }
+
+func TestSyslogTimestampParser(t *testing.T) {
+	p := &Parser{
+		Patterns: []string{`%{SYSLOGTIMESTAMP:timestamp:ts-syslog} value=%{NUMBER:value:int}`},
+		timeFunc: func() time.Time { return time.Date(2018, time.April, 1, 0, 0, 0, 0, nil) },
+	}
+	require.NoError(t, p.Compile())
+	m, err := p.ParseLine("Sep 25 09:01:55 value=42")
+	require.NoError(t, err)
+	require.NotNil(t, m)
+	require.Equal(t, 2018, m.Time().Year())
+}