Fix shutdown order for collector components

[Sreevani871]

bug fix: Sequence of shutdown calls resulting in crash due to panic and leads to span loss to avoid span loss

Which problem is this PR solving?

• Resolves Collector: Sequence of shutdown calls resulting in crash due to panic and leads to span loss #2379
• The clean shutdown of collector service without span loss and crash due to panic.

Short description of the changes

• Changes include reordering of the shutdown sequence as follows to allow collector to stop accepting new spans and drain the queue to avoid span loss.
  servers -> queue processors (with drain) -> writers

[codecov]

Codecov Report

Merging #2381 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2381   +/-   ##
=======================================
  Coverage   95.60%   95.61%           
=======================================
  Files         206      206           
  Lines       10548    10548           
=======================================
+ Hits        10084    10085    +1     
+ Misses        396      395    -1     
  Partials       68       68           

Impacted Files	Coverage Δ
cmd/flags/admin.go	77.77% <0.00%> (-1.59%)	⬇️
...lugin/sampling/strategystore/adaptive/processor.go	100.00% <0.00%> (+0.79%)	⬆️

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[jpkrohling]

LGTM!