Service discovery refactor

[owen-d]

Mimics how Prometheus used to reference scrape configs & adds more tests.

[codecov-io]

Codecov Report

Merging #2761 into master will decrease coverage by 0.16%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master    #2761      +/-   ##
==========================================
- Coverage   61.68%   61.51%   -0.17%     
==========================================
  Files         171      172       +1     
  Lines       13389    13395       +6     
==========================================
- Hits         8259     8240      -19     
- Misses       4378     4402      +24     
- Partials      752      753       +1     

Impacted Files	Coverage Δ
pkg/promtail/scrapeconfig/scrapeconfig.go	37.50% <0.00%> (ø)
pkg/promtail/targets/file/filetargetmanager.go	0.00% <0.00%> (ø)
pkg/promtail/targets/stdin/stdin_target_manager.go	71.69% <100.00%> (-0.53%)	⬇️
pkg/promtail/positions/positions.go	46.80% <0.00%> (-11.71%)	⬇️
pkg/promtail/targets/file/tailer.go	70.09% <0.00%> (-5.61%)	⬇️
pkg/promtail/targets/file/filetarget.go	64.08% <0.00%> (-2.12%)	⬇️
pkg/promtail/config/config.go	0.00% <0.00%> (ø)
pkg/querier/queryrange/downstreamer.go	97.64% <0.00%> (+2.35%)	⬆️

[slim-bean]

LGTM