Decouple logql engine/AST from execution context

[owen-d]

What

This PR introduces a few changes/refactorings designed to decouple the LogQL engine and AST from its execution context. The idea is that an AST is just a stateless tree structure which can be processed by an evaluation engine. Separating the evaluation from AST will allow us to more easily build different evaluation contexts (sharding comes to mind).

• logql Engine is now an interface
• iterators and evaluators are no longer struct fields on AST nodes
• introduces an Evaluator interface
• evaluators can be combined with ast nodes in order to resolve the underlying queries.

Why

Previously, evaluators and iterators were tied directly into the AST nodes themselves. This makes them difficult to extend/refactor/etc.

$ benchcmp /tmp/master.out /tmp/refactor.out
benchmark                        old ns/op      new ns/op      delta
BenchmarkContainsFilter-8        8.30           8.41           +1.33%
BenchmarkRangeQuery100000-8      2516982        2213231        -12.07%
BenchmarkRangeQuery200000-8      10473399       12376741       +18.17%
BenchmarkRangeQuery500000-8      1937572837     1888996274     -2.51%
BenchmarkRangeQuery1000000-8     4224820576     3931777379     -6.94%

benchmark                        old allocs     new allocs     delta
BenchmarkRangeQuery100000-8      2848           2907           +2.07%
BenchmarkRangeQuery200000-8      3333           3514           +5.43%
BenchmarkRangeQuery500000-8      136397         136449         +0.04%
BenchmarkRangeQuery1000000-8     269751         269815         +0.02%

benchmark                        old bytes     new bytes     delta
BenchmarkRangeQuery100000-8      412186        405320        -1.67%
BenchmarkRangeQuery200000-8      894959        1009403       +12.79%
BenchmarkRangeQuery500000-8      135070672     135033424     -0.03%
BenchmarkRangeQuery1000000-8     270658240     270658704     +0.00%

[codecov-io]

Codecov Report

Merging #1605 into master will increase coverage by 0.09%.
The diff coverage is 89.45%.

@@            Coverage Diff             @@
##           master    #1605      +/-   ##
==========================================
+ Coverage   60.65%   60.75%   +0.09%     
==========================================
  Files         106      107       +1     
  Lines        8052     8075      +23     
==========================================
+ Hits         4884     4906      +22     
- Misses       2783     2784       +1     
  Partials      385      385

Impacted Files	Coverage Δ
pkg/querier/http.go	7.06% <0%> (ø)	⬆️
pkg/querier/querier.go	70.06% <100%> (ø)	⬆️
pkg/logql/ast.go	84.37% <69.23%> (-1%)	⬇️
pkg/logql/engine.go	87.78% <86.04%> (-3.75%)	⬇️
pkg/logql/evaluator.go	92.78% <92.78%> (ø)
pkg/promtail/targets/filetarget.go	70.55% <0%> (+1.84%)	⬆️
pkg/promtail/targets/tailer.go	75.86% <0%> (+2.29%)	⬆️

[cyriltovena]

LGTM