Clearer error response from push endpoint when labels are malformed

[talham7391]

What this PR does / why we need it:
Currently, when users use the /loki/api/v1/push with a malformed labels, they get an error response saying something along the lines of:
parse error at line 1, col 4: literal not terminated.

It isn't clear that the labels are the issue. This PR makes it clear.

Which issue(s) this PR fixes:
Fixes #1727

Checklist

• Documentation added
• Tests updated

[claassistantio]

All committers have signed the CLA.

[talham7391]

I signed the agreement, made sure my emails are added to my account as well. Not sure why it says I haven't signed it 🤔

[slim-bean]

Hi @talham7391 thanks for the PR!

It looks like the email associated with the commit is not valid and therefore github can't link it to your account.

You would need to git config --global user.email "email@example.com" with an email associated to your github account and re-write the commit/force push back to the PR I think for the CLA to be happy.

[talham7391]

Thanks! @slim-bean

[codecov-io]

Codecov Report

Merging #1750 into master will decrease coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1750      +/-   ##
==========================================
- Coverage   64.33%   64.28%   -0.05%     
==========================================
  Files         121      121              
  Lines        9081     9081              
==========================================
- Hits         5842     5838       -4     
- Misses       2835     2837       +2     
- Partials      404      406       +2

Impacted Files	Coverage Δ
pkg/distributor/validator.go	82.6% <100%> (ø)	⬆️
pkg/ingester/transfer.go	65% <0%> (-1.43%)	⬇️
pkg/logql/evaluator.go	91.22% <0%> (-0.59%)	⬇️

[cyriltovena]

LGTM just a small suggestion if you have time.