perf(chstorage): do not make an unnecessary copy of encoded attributes

[tdakkota]

For #293.

                 │   old.txt    │               new.txt               │
                 │    sec/op    │   sec/op     vs base                │
_metricsBatch-32   1059.2µ ± 0%   824.0µ ± 1%  -22.21% (p=0.000 n=15)

                 │    old.txt     │               new.txt                │
                 │      B/op      │     B/op      vs base                │
_metricsBatch-32   290.716Ki ± 0%   4.317Ki ± 0%  -98.51% (p=0.000 n=15)

                 │   old.txt    │              new.txt               │
                 │  allocs/op   │ allocs/op   vs base                │
_metricsBatch-32   6280.00 ± 0%   16.00 ± 0%  -99.75% (p=0.000 n=15)

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (401f437) 72.96% compared to head (d06609c) 72.99%.
Report is 22 commits behind head on main.

Files	Patch %	Lines
internal/chstorage/inserter_metrics.go	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #305      +/-   ##
==========================================
+ Coverage   72.96%   72.99%   +0.03%     
==========================================
  Files         178      178              
  Lines       13884    13881       -3     
==========================================
+ Hits        10130    10132       +2     
+ Misses       3254     3249       -5     
  Partials      500      500              

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