Use approximate memory usage for QueueInfo

[hibiken]

Closes #309

[codecov]

Codecov Report

Merging #310 (7169c56) into master (bff2a05) will decrease coverage by 0.01%.
The diff coverage is 75.00%.

❗ Current head 7169c56 differs from pull request most recent head 3306d24. Consider uploading reports for the commit 3306d24 to get more accurate results

@@            Coverage Diff             @@
##           master     #310      +/-   ##
==========================================
- Coverage   73.18%   73.17%   -0.02%     
==========================================
  Files          20       20              
  Lines        2614     2613       -1     
==========================================
- Hits         1913     1912       -1     
  Misses        494      494              
  Partials      207      207              

Impacted Files	Coverage Δ
inspector.go	57.42% <ø> (ø)
internal/rdb/inspect.go	68.15% <75.00%> (-0.06%)	⬇️

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