[MOD-13819] Add two new metrics for tiered index observability: frontend buffer size and HNSW main thread insertion

[alonre24]

Describe the changes in the pull request

Added two metrics for the statistics info (relevant only for tiered index). See spec in RediSearch/RediSearch#8229)

Mark if applicable

• This PR introduces API changes
• This PR introduces serialization changes

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.09%. Comparing base (3e94722) to head (4ff06e8).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #901      +/-   ##
==========================================
- Coverage   97.11%   97.09%   -0.03%     
==========================================
  Files         129      129              
  Lines        7493     7500       +7     
==========================================
+ Hits         7277     7282       +5     
- Misses        216      218       +2     

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[lerman25]

Nice!
Small comment