Preallocate and keep memory for HashMap in Metric aggregation

[cijothomas]

Discussed originally in #2267 (comment)
and in #2288 (comment)

2 changes

1. HashMap for aggregations are created with enough capacity to hold cardinality. (Its currently hardcoded, but will be configurable later)
2. Delta - during collect, the hashmap is swapped with a secondary hashmap., and the original is drained.

These ensures HashMap never requires re-sizing in its lifetime.

Incremental changes only, as this is not quite the final state - we'll need to likely store the overflow outside of hashmap and also tackle #2328

[codecov]

Codecov Report

Attention: Patch coverage is 82.35294% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.5%. Comparing base (9a85ce1) to head (759f2a7).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/internal/mod.rs	81.2%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2343     +/-   ##
=======================================
- Coverage   79.5%   79.5%   -0.1%     
=======================================
  Files        123     123             
  Lines      21365   21371      +6     
=======================================
+ Hits       17004   17008      +4     
- Misses      4361    4363      +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.