Add multi-line detection support to promtail

[davidhao3300]

Relevant issue: #74

Largely inspired by the code in the fluentd concat plugin: https://github.com/fluent-plugins-nursery/fluent-plugin-concat

Adds a buffer in front of the HTTP client that checks whether a line matches a regex. Lines that do not are merged with a newline with buffered lines. When a new log line matches the regex, the previous lines are flushed, and the new line is buffered. A task that runs every second checks buffered lines and force-flushes them if they're older than a configured timeout.

Regex and timeout are configurable per-scraper-config. By default, the concat functionality is disabled to preserve existing behavior.

TODOs:

• docs (will do after the overall idea of this PR is okay'd
• verify that configuration through YAML actually works
• determine whether we should keep this disabled by default

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[bryanyork]

@davidhao3300 Do you plan on continuing here?

[davidhao3300]

I'd say probably not, but feel free to use any code in this branch!