Move time from DataPoint to Histogram/ExpoHistogram

[fraillt]

Fixes #1566.

Changes

Move time and start_time fields from DataPoint to specific aggregations, - Histogram and ExponentialHistogram.

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 71.42857% with 22 lines in your changes missing coverage. Please review.

Project coverage is 79.2%. Comparing base (1a4e931) to head (c105ed6).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-stdout/src/metrics/exporter.rs	0.0%	12 Missing ⚠️
opentelemetry-proto/src/transform/metrics.rs	0.0%	4 Missing ⚠️
...-sdk/src/metrics/internal/exponential_histogram.rs	86.6%	4 Missing ⚠️
...etry-sdk/src/testing/metrics/in_memory_exporter.rs	66.6%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2411     +/-   ##
=======================================
- Coverage   79.3%   79.2%   -0.1%     
=======================================
  Files        122     122             
  Lines      21511   21516      +5     
=======================================
- Hits       17060   17059      -1     
- Misses      4451    4457      +6     

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