Fix loki log output exiting before push is finished

[mstoykov]

After #2833 log outputs were stopped but loki didn't wait until it push was finished to signal it was done.

Which leads to the actual k6 process exiting before loki could flush its messages.

[mstoykov]

I did use a sync.WaitGroup in thePoC, but decided there is no need to import the sync package and this code is even shorter so 🤷

[codebien]

I know this code is complex and maybe not so easy, but it would be good to have a fixed test that fails without this patch.

Up to you if you have time for it.

[codecov-commenter]

Codecov Report

Merging #3194 (11f3af5) into master (ea3a23d) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

❗ Current head 11f3af5 differs from pull request most recent head 3eaf6f6. Consider uploading reports for the commit 3eaf6f6 to get more accurate results

@@            Coverage Diff             @@
##           master    #3194      +/-   ##
==========================================
- Coverage   72.88%   72.85%   -0.04%     
==========================================
  Files         256      256              
  Lines       19800    19803       +3     
==========================================
- Hits        14431    14427       -4     
- Misses       4469     4475       +6     
- Partials      900      901       +1     

Flag	Coverage Δ
ubuntu	72.78% <0.00%> (-0.05%)	⬇️
windows	72.68% <0.00%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
log/loki.go	30.35% <0.00%> (-0.36%)	⬇️

... and 2 files with indirect coverage changes

[mstoykov]

a fixed test that fails without this patch.

My original code was fixing the problem in totally different place, which was going to make the test .... really hard to write. But later I found out that ... the fix both didn't work and I didn't need to make it outside as the bug was actually in loki log output to begin with.

I am going to try to figure out if this is fairly easy to fix.