chore: remove mem table memory metrics

[wcy-fdu]

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What's changed and what's your intention?

As mem table is spillable, the memory usage of it is unused.

Resolves #13595

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[fuyufjh]

Actually I slightly tend to keep such metrics just in case. But considering this implementation looks heavy, it's okay to me to just remove it.

[wcy-fdu]

Actually I slightly tend to keep such metrics just in case. But considering this implementation looks heavy, it's okay to me to just remove it.

At first I had similar thoughts, but I realized that this(histogram) metrcis is hardly observable under normal circumstances, and when barrier stuck, mem table will spill, so I decided to remove it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b3eecb3) 68.16% compared to head (7977a5a) 68.10%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13604      +/-   ##
==========================================
- Coverage   68.16%   68.10%   -0.07%     
==========================================
  Files        1512     1512              
  Lines      261162   261144      -18     
==========================================
- Hits       178034   177854     -180     
- Misses      83128    83290     +162     

Flag	Coverage Δ
rust	68.10% <ø> (-0.07%)	⬇️

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