Better logql metric status code.

[cyriltovena]

This PR improves 3 areas:

• Better status code for logql metrics.
• More granular histogram based on historical data.
• Querier now returns 400 only when there is a parsing error in the logql engine.

Fixes #1658

[codecov-io]

Codecov Report

Merging #1718 into master will decrease coverage by 0.05%.
The diff coverage is 58.82%.

@@            Coverage Diff            @@
##           master   #1718      +/-   ##
=========================================
- Coverage   64.16%   64.1%   -0.06%     
=========================================
  Files         121     121              
  Lines        9024    9069      +45     
=========================================
+ Hits         5790    5814      +24     
- Misses       2829    2851      +22     
+ Partials      405     404       -1

Impacted Files	Coverage Δ
pkg/logql/ast.go	88.18% <0%> (ø)	⬆️
pkg/loki/modules.go	11.29% <0%> (ø)	⬆️
pkg/querier/http.go	6.73% <0%> (-0.33%)	⬇️
pkg/querier/queryrange/roundtrip.go	80.43% <100%> (ø)	⬆️
pkg/logql/engine.go	90.06% <25%> (-1.14%)	⬇️
pkg/logql/metrics.go	80.64% <50%> (-5.07%)	⬇️
pkg/querier/queryrange/stats.go	82.75% <78.94%> (-17.25%)	⬇️
pkg/logql/parser.go	82% <83.33%> (-0.23%)	⬇️
pkg/querier/queryrange/codec.go	92.65% <0%> (-0.82%)	⬇️
pkg/promtail/targets/filetarget.go	70.55% <0%> (+1.84%)	⬆️
... and 1 more

[joe-elliott]

LGTM