Not 100% sure about this, but we're recording `@captured_at` which records when the log was originally scraped from the file.

How about only filling in the timestamp with `Time.parse($1)` if `@timestamp` == `@captured_at`.

i.e. if the timestamp is that of when it was scraped from the file, then do one better and replace it, otherwise presume whatever filled in `@timestamp` knows better? Then there is no need for the awkward `:parse_ts` option?