loki: Fixed out of order entries allowed in a chunk on edge case

[pracucci]

What this PR does / why we need it:
A chunk can contain log entries out of order (leading also to incorrect values returned by .Bounds()) if an out of order log entry is pushed as first entry of a new block, right after the previous block has been cut.

Checklist

• Documentation added
• Tests updated

[cyriltovena]

LGTM

[cyriltovena]

How did you figured this was happening ?

[slim-bean]

@pracucci nice find!

When you have a chance can you fix the few linter issues:

pkg/chunkenc/gzip_test.go:236:11: Error return value of `chk.cut` is not checked (errcheck)
			chk.cut()
			       ^
pkg/chunkenc/gzip_test.go:242:11: Error return value of `chk.cut` is not checked (errcheck)
			chk.cut()
			       ^
pkg/chunkenc/gzip_test.go:245:11: Error return value of `chk.cut` is not checked (errcheck)
			chk.cut()
			       ^

[pracucci]

How did you figured this was happening ?

@cyriltovena I was reviewing the chunk implementation and noticed it. Then I tried to manually reproduce it pushing an out of order log with promtail right after cutting a block, and was reproduced.

can you fix the few linter issues

Sure, done! Yesterday I've pushed the PR and went to sleep without waiting for the CI result.