Improve object key parsing for boltdb shipper.

[cyriltovena]

Should help discover what's going on in #2315

Signed-off-by: Cyril Tovena cyril.tovena@gmail.com

[codecov-commenter]

Codecov Report

Merging #2323 into master will increase coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2323      +/-   ##
==========================================
+ Coverage   61.16%   61.20%   +0.04%     
==========================================
  Files         158      158              
  Lines       12799    12811      +12     
==========================================
+ Hits         7828     7841      +13     
- Misses       4374     4376       +2     
+ Partials      597      594       -3     

Impacted Files	Coverage Δ
pkg/storage/stores/local/downloads.go	0.00% <0.00%> (ø)
pkg/logql/evaluator.go	92.13% <0.00%> (ø)
pkg/promtail/targets/file/filetarget.go	70.48% <0.00%> (+1.80%)	⬆️
pkg/promtail/targets/file/tailer.go	78.40% <0.00%> (+4.54%)	⬆️
pkg/logql/vector.go	87.50% <0.00%> (+18.75%)	⬆️

[owen-d]

lgtm