Timestamp stage add a fudged timestamp for lines missing a timestamp

[slim-bean]

If using the timestamp stage and processing multi-line log lines, (or log lines that might not have a timestamp), the behavior could be a bit unpredictable.

If the timestamp stage fails to match the timestamp, the promtail system time will be used for the log line, this creates a race between the last processed log lines timestamp and the system time, potentially leading to Loki rejecting the log line which had no timestamp as being older than the previous, or possibly the next log line with a timestamp being rejected if it's timestamp is older than what the system time applied was.

I am suggesting an option be added to the timestamp stage to allow fudging a timestamp onto log lines which do not have one. The stage should keep track of the previous lines timestamp and if the next line coming in doesn't have a timestamp, increment the previous timestamp by one nanosecond and use that as the timestamp.

Fudging a timestamp is not great and could potentially lead to problems however, with nanosecond precision I would hope this to be very unlikely.

I also think the default for this behavior should be enabled, with an option to disable it

[pracucci]

I'm working on it.