Add support for the development per-series starttime feature

[dashpole]

Add a feature to use per-series start times to match the spec: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#start-timestamps

This is a prerequisite to finishing / closing.

Previous prototype: #7719

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.7%. Comparing base (768e930) to head (0a71705).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8060   +/-   ##
=====================================
  Coverage   81.7%   81.7%           
=====================================
  Files        308     308           
  Lines      23606   23628   +22     
=====================================
+ Hits       19296   19316   +20     
- Misses      3927    3929    +2     
  Partials     383     383           

Files with missing lines	Coverage Δ
sdk/internal/x/features.go	100.0% <100.0%> (ø)
...metric/internal/aggregate/exponential_histogram.go	98.7% <100.0%> (+<0.1%)	⬆️
sdk/metric/internal/aggregate/histogram.go	100.0% <100.0%> (ø)
sdk/metric/internal/aggregate/lastvalue.go	100.0% <100.0%> (ø)
sdk/metric/internal/aggregate/sum.go	100.0% <100.0%> (ø)

... and 5 files with indirect coverage changes

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