Fix potential overflow in ledger metrics LatestTrieRegCountDiff() and LatestTrieMaxDepthDiff()

[fxamacker]

Changes

Prevent overflow by replacing uint64 with int64 in two functions APIs that can receive negative deltas:

• LatestTrieRegCountDiff(number int64)
• LatestTrieMaxDepthDiff(number int64)

Also cast potentially overflowing expressions calling these functions.

Closes #2092

Caveat

This is a minimal fix that doesn't prevent all overflow scenarios. For example, if the delta doesn't fit into int64 then we still have an overflow but that scenario is unlikely given the size of int64.

[codecov-commenter]

Codecov Report

Merging #2096 (658545f) into master (adade03) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2096      +/-   ##
==========================================
+ Coverage   57.28%   57.29%   +0.01%     
==========================================
  Files         634      634              
  Lines       36555    36555              
==========================================
+ Hits        20939    20946       +7     
+ Misses      12985    12978       -7     
  Partials     2631     2631              

Flag	Coverage Δ
unittests	57.29% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
ledger/complete/mtrie/forest.go	61.14% <100.00%> (ø)
consensus/hotstuff/eventloop/event_loop.go	68.29% <0.00%> (ø)
...sus/approvals/assignment_collector_statemachine.go	47.11% <0.00%> (+6.73%)	⬆️

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