More semantically correct flush shutdown

[owen-d]

The endpoint is now a bit more explicit that it flushes then shuts down.

[slim-bean]

👍

[codecov-io]

Codecov Report

Merging #3146 (97debb7) into master (b72198c) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #3146   +/-   ##
=======================================
  Coverage   63.18%   63.19%           
=======================================
  Files         188      188           
  Lines       16305    16305           
=======================================
+ Hits        10303    10304    +1     
+ Misses       5063     5060    -3     
- Partials      939      941    +2     

Impacted Files	Coverage Δ
pkg/loki/modules.go	3.25% <0.00%> (ø)
pkg/promtail/targets/file/tailer.go	72.97% <0.00%> (-5.41%)	⬇️
pkg/querier/queryrange/downstreamer.go	95.29% <0.00%> (-2.36%)	⬇️
pkg/canary/comparator/comparator.go	76.36% <0.00%> (-1.82%)	⬇️
pkg/logql/evaluator.go	90.23% <0.00%> (+0.35%)	⬆️
pkg/promtail/positions/positions.go	58.51% <0.00%> (+11.70%)	⬆️