feat: csv parsers can handle unix timestamp

If we try to use unix timestamp in the csv file, we will get an error.
This commit fixes that.
So you can have timestamp like : 1405544146 now.

plugins/parsers/csv/parser.go

@@ -220,7 +220,12 @@ outer:
 		var err error
 		metricTime, err = time.Parse(p.TimestampFormat, tStr)
 		if err != nil {
-			return nil, err
+			if t, errT := strconv.ParseInt(tStr, 10, 64); errT == nil {
+				// Try Unix format
+				metricTime = time.Unix(t, 0)
+			} else {
+				return nil, err
+			}
 		}
 	}