compiler guarantees for logql exprs

[owen-d]

This PR ensures that the logql.Expr interface is not implemented accidentally. This is helpful for understanding its source code and will generate compiler errors if a noncompliant type is used as an Expr, which will gives us more assurance in the safety of our code moving forward.

[codecov-io]

Codecov Report

Merging #1626 into master will decrease coverage by 0.09%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master    #1626     +/-   ##
=========================================
- Coverage   60.95%   60.86%   -0.1%     
=========================================
  Files         108      108             
  Lines        8144     8148      +4     
=========================================
- Hits         4964     4959      -5     
- Misses       2790     2796      +6     
- Partials      390      393      +3

Impacted Files	Coverage Δ
pkg/logql/ast.go	81.81% <0%> (-2.56%)	⬇️
pkg/promtail/targets/filetarget.go	68.71% <0%> (-1.85%)	⬇️
pkg/logql/evaluator.go	91.75% <0%> (-1.04%)	⬇️
pkg/promtail/targets/tailer.go	73.56% <0%> (ø)	⬆️

[cyriltovena]

LGTM