PoC: Metric instruments support an WithOptIn option.

[dashpole]

The SDK uses the DropAggregation unless it is enabled using a view.

[codecov]

Codecov Report

❌ Patch coverage is 98.14815% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.8%. Comparing base (fa89e4c) to head (cf2d56b).
⚠️ Report is 74 commits behind head on main.

Files with missing lines	Patch %	Lines
sdk/metric/pipeline.go	81.8%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #7721     +/-   ##
=======================================
- Coverage   86.2%   81.8%   -4.5%     
=======================================
  Files        302     304      +2     
  Lines      21991   23328   +1337     
=======================================
+ Hits       18969   19087    +118     
- Misses      2641    3858   +1217     
- Partials     381     383      +2     

Files with missing lines	Coverage Δ
metric/asyncfloat64.go	100.0% <100.0%> (ø)
metric/asyncint64.go	100.0% <100.0%> (ø)
metric/instrument.go	100.0% <100.0%> (ø)
metric/syncfloat64.go	100.0% <100.0%> (ø)
metric/syncint64.go	100.0% <100.0%> (ø)
sdk/metric/instrument.go	97.5% <ø> (ø)
sdk/metric/meter.go	92.1% <100.0%> (ø)
sdk/metric/view.go	100.0% <100.0%> (ø)
sdk/metric/pipeline.go	89.6% <81.8%> (-0.6%)	⬇️

... and 11 files with indirect coverage changes

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[dashpole]

I updated this PoC by changing DefaultDisabled to OptIn, and by adding the new view stream Enabled field.