vfs: merge fuse ops metrics in .stats

[SandyXSD]

A patch for #4554 .
Without it there will be multiple lines for fuse ops metrics:

juicefs_fuse_ops_durations_histogram_seconds_total 101
juicefs_fuse_ops_durations_histogram_seconds_sum 1.305467913
juicefs_fuse_ops_durations_histogram_seconds_total 205
juicefs_fuse_ops_durations_histogram_seconds_sum 1.6931756689999993
juicefs_fuse_ops_durations_histogram_seconds_total 211
juicefs_fuse_ops_durations_histogram_seconds_sum 0.05859979800000001
juicefs_fuse_ops_durations_histogram_seconds_total 315
juicefs_fuse_ops_durations_histogram_seconds_sum 0.09348225000000002

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.72%. Comparing base (7de5d50) to head (8b922b7).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4560      +/-   ##
==========================================
- Coverage   55.72%   55.72%   -0.01%     
==========================================
  Files         157      157              
  Lines       42968    42969       +1     
==========================================
- Hits        23946    23944       -2     
- Misses      16364    16366       +2     
- Partials     2658     2659       +1     

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