algod : deregister metrics tracker on close

[algorandskiy]

Summary

Metrics counters where not cleared on close and lead to duplicate entries in metering report.

Test Plan

Added unit test. Tested manually.

Closes #3354

[codecov-commenter]

Codecov Report

Merging #3661 (fad2a91) into master (468d34b) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3661      +/-   ##
==========================================
+ Coverage   48.12%   48.13%   +0.01%     
==========================================
  Files         381      381              
  Lines       62214    62223       +9     
==========================================
+ Hits        29939    29950      +11     
+ Misses      28847    28844       -3     
- Partials     3428     3429       +1     

Impacted Files	Coverage Δ
ledger/metrics.go	100.00% <100.00%> (ø)
crypto/merkletrie/trie.go	66.42% <0.00%> (-2.19%)	⬇️
crypto/merkletrie/node.go	91.62% <0.00%> (-1.87%)	⬇️
cmd/tealdbg/debugger.go	71.42% <0.00%> (-0.99%)	⬇️
network/wsPeer.go	68.33% <0.00%> (-0.56%)	⬇️
catchup/service.go	68.88% <0.00%> (+0.24%)	⬆️
network/wsNetwork.go	63.09% <0.00%> (+0.29%)	⬆️
network/requestTracker.go	71.12% <0.00%> (+0.86%)	⬆️
data/transactions/verify/txn.go	45.02% <0.00%> (+0.86%)	⬆️
ledger/blockqueue.go	85.05% <0.00%> (+2.87%)	⬆️

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[tsachiherman]

I'm not opposing to this change, but please make sure to look into Registry.Deregister; it looks like it not thread-safe.

[algorandskiy]

close() is under l.trackerMu lock. And noone else deregisters any counter.
I guess making the registry thread safe is a separate issue.

[algorandskiy]

Looks like Deregister simply missed the mutex op since the mutex does exist in Registry and used in Register. Fixed.