Fixes path prefix in the querier.

[cyriltovena]

This solves the problem of path prefix in the querier only see #2176

For the frontend we need to fix cortex first, then we can also fix it in Loki.

At least with this PR people using helm and the single binary won't have this issue.

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

[codecov-commenter]

Codecov Report

Merging #2178 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2178   +/-   ##
=======================================
  Coverage   61.58%   61.58%           
=======================================
  Files         154      154           
  Lines       12310    12310           
=======================================
  Hits         7581     7581           
+ Misses       4121     4120    -1     
- Partials      608      609    +1     

Impacted Files	Coverage Δ
pkg/loghttp/versions.go	100.00% <100.00%> (ø)
pkg/logql/evaluator.go	90.02% <0.00%> (-0.54%)	⬇️
pkg/querier/queryrange/downstreamer.go	97.93% <0.00%> (+2.06%)	⬆️