promtail: support unix timestamps with fractional seconds

[flixr]

Is your feature request related to a problem? Please describe.

Basically all of our logs contain timestamps like 1591375232, 1591375232.833 or 1591375232.833587995 (in the same logfile) and it would be a pity to loose the fractions.

Describe the solution you'd like
The Unix format in the timestamp stage should also accept fractional seconds.

Describe alternatives you've considered
Current alternative is to write a regex that only extracts the full seconds, but this is clearly not very nice.

Additional context

Maybe something like this in util.go:

	case "Unix":
		return func(t string) (time.Time, error) {
			if strings.Contains(t, ".") {
				f, err := strconv.ParseFloat(t, 64)
				if err != nil {
					return time.Time{}, err
				}
				sec, dec := math.Modf(f)
				return time.Unix(int64(sec), int64(dec*(1e9))), nil
			}
			i, err := strconv.ParseInt(t, 10, 64)
			if err != nil {
				return time.Time{}, err
			}
			return time.Unix(i, 0), nil
		}