Fix panic in collector when Zipkin server is shutdown

[Sreevani871]

…cit shutdown

Signed-off-by: Sreevani871 sreevani@freshdesk.com

Which problem is this PR solving?

• Solving Collector exiting before shutdown process complete due to Fatal error #2395

Short description of the changes

• Handling explicit shutdown case to avoid collector exiting abruptly after Zipkin server close to allow shutdown process to complete.

[codecov]

Codecov Report

Merging #2392 into master will increase coverage by 0.01%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master    #2392      +/-   ##
==========================================
+ Coverage   95.61%   95.62%   +0.01%     
==========================================
  Files         206      206              
  Lines       10547    10549       +2     
==========================================
+ Hits        10084    10088       +4     
+ Misses        395      394       -1     
+ Partials       68       67       -1     

Impacted Files	Coverage Δ
cmd/collector/app/server/zipkin.go	0.00% <0.00%> (ø)
cmd/collector/app/collector.go	69.56% <100.00%> (+0.44%)	⬆️
plugin/storage/integration/integration.go	80.38% <0.00%> (-0.48%)	⬇️
...lugin/sampling/strategystore/adaptive/processor.go	100.00% <0.00%> (+0.79%)	⬆️
cmd/query/app/static_handler.go	85.83% <0.00%> (+1.66%)	⬆️

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[yurishkuro]

Thanks!