Option to drop/ignore lines that don't match a given expression.

[mzs114]

Is your feature request related to a problem? Please describe.
Sometimes the user exactly wants lines that match a particular expression be captured and others ignored/dropped, those lines could be noise/irrelevant for the end user. And/or there are too many patterns in the files and the user is interested in specific one(s).

Describe the solution you'd like
A configuration option(like in packet filters) to drop/ignore lines that don't match the expression in pipeline_stages.

Describe alternatives you've considered
Hypothetically(As loki and promtail don't have #553 yet), write more rules/stages to drop lines with various patterns, if there are multiple patterns to be ignore then this increases the effot for the end user and becomes complex.

Additional context
There are other logging patterns, like that start with '#' as the beginning char(MS IIS) and others which usually include multiline comments in debug levels(Zend PHP server, commercial). In all these cases the user would probably prefer to capture a particular pattern rather than write multi stage pipeline to drop the N patterns.