Add collect methods on ValueMap

[fraillt]

Design discussion issue #2145

Changes

Mostly extract DataPoint collection logic from specific metrics implementations into ValueMap under collect_readonly and collect_and_reset functions.
I tried to preserve existing logic with few exceptions:

• for cumulative temporality - acquire read lock on hashmap instead of write lock.
• for delta temporality - since hashmap is cleared anyway, reduce the time hashmap is locked. Instead of "lock->iterate(consume)->unlock" changed to "lock->consume->unlock->iterate".

There are a lot more things that could be improved, but with this change it will become more convenient/simpler to experiment/implement further improvements in the future.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 98.52941% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.4%. Comparing base (1fd871a) to head (fc04691).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/internal/mod.rs	95.4%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2267     +/-   ##
=======================================
- Coverage   79.5%   79.4%   -0.2%     
=======================================
  Files        121     121             
  Lines      20978   20787    -191     
=======================================
- Hits       16685   16512    -173     
+ Misses      4293    4275     -18     

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

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: fraillt / name: Mindaugas Vinkelis (8d56bd2, 66cb25a)
• ✅ login: cijothomas / name: Cijo Thomas (f343aa0, fc04691, e52baeb, e99bb97)
• ✅ login: lalitb / name: Lalit Kumar Bhasin (edbbeaf)

[utpilla]

Great work!

There are two newer kinds of allocations happening with these changes as mentioned in #2267 (comment) and #2267 (comment). These can be avoided. I had a chat with @fraillt on Slack and he agreed to address these in his upcoming PRs.

[cijothomas]

Thanks! Looking forward to the follow up improvements.