lotus-stats: insert miner and network power data as gibibytes to avoid int64 overflows

[travisperson]

Resolves #6920.

[codecov]

Codecov Report

Merging #7194 (0e8abc1) into master (7c484ca) will increase coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #7194      +/-   ##
==========================================
+ Coverage   34.71%   34.73%   +0.02%     
==========================================
  Files         685      685              
  Lines       80198    80199       +1     
==========================================
+ Hits        27841    27858      +17     
+ Misses      46678    46667      -11     
+ Partials     5679     5674       -5     

Impacted Files	Coverage Δ
tools/stats/metrics.go	0.00% <0.00%> (ø)
chain/actors/builtin/market/v4.go	36.52% <0.00%> (-7.83%)	⬇️
extern/sector-storage/sched_worker.go	77.25% <0.00%> (-0.73%)	⬇️
chain/store/store.go	64.47% <0.00%> (+0.71%)	⬆️
storage/wdpost_changehandler.go	98.58% <0.00%> (+0.94%)	⬆️
extern/storage-sealing/fsm.go	56.62% <0.00%> (+1.20%)	⬆️
itests/kit/blockminer.go	95.23% <0.00%> (+1.58%)	⬆️
storage/wdpost_sched.go	81.18% <0.00%> (+1.98%)	⬆️
chain/sub/incoming.go	52.07% <0.00%> (+2.36%)	⬆️
miner/miner.go	54.30% <0.00%> (+2.64%)	⬆️

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