[SDK] Set small initial value for AttributesHashMap::hash_map_

[ThomsonTan]

Fixes #3390

Changes

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.05%. Comparing base (1e33bb5) to head (6821f60).
Report is 46 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3391   +/-   ##
=======================================
  Coverage   90.05%   90.05%           
=======================================
  Files         212      212           
  Lines        6932     6932           
=======================================
  Hits         6242     6242           
  Misses        690      690           

Files with missing lines	Coverage Δ
...entelemetry/sdk/metrics/state/attributes_hashmap.h	96.37% <100.00%> (ø)

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[ThomsonTan]

The initial large bucket count for attribute hash map could be mitigated by recording multiple dimensions to one instruments. I think it is still rare to have only one or two attribute sets on an instrument.

Let me move this to draft and do some more experiment on it.

[ThomsonTan]

More benchmark needed for this tuning, close it for now.