Prototype for the advice api

[dashpole]

This demonstrates that an option on histogram instruments for providing explicit bucket boundaries can be added in a backwards-compatible manner.

This prototype is missing:

• correct naming ("advice" should be here somewhere, but it seemed clunky), so I skipped it.
• this is not a field on "Instrument", as required by the specification. That would seem to require our Instrument struct to export an Advice field. But our Instrument struct is used by Views, and it seems incorrect to select on advice from a view.
• I didn't make the bucket boundaries identifying, as required by the spec. It would be easy to do that in this prototype by adding the buckets to instID. See Ensure the advice API can be added to the metric SDK #4162 (comment)
• Tests beyond a single unit test

Prototype for #4162

[codecov]

Codecov Report

Merging #4341 (9915c30) into main (e08359f) will decrease coverage by 0.1%.
The diff coverage is 77.5%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4341     +/-   ##
=======================================
- Coverage   83.4%   83.3%   -0.1%     
=======================================
  Files        185     185             
  Lines      14332   14354     +22     
=======================================
+ Hits       11958   11971     +13     
- Misses      2147    2156      +9     
  Partials     227     227             

Impacted Files	Coverage Δ
metric/instrument.go	95.1% <0.0%> (-4.9%)	⬇️
metric/syncfloat64.go	93.7% <0.0%> (-6.3%)	⬇️
metric/syncint64.go	93.7% <0.0%> (-6.3%)	⬇️
sdk/metric/instrument.go	92.1% <100.0%> (+0.3%)	⬆️
sdk/metric/meter.go	85.9% <100.0%> (+<0.1%)	⬆️
sdk/metric/pipeline.go	91.3% <100.0%> (+0.1%)	⬆️

... and 1 file with indirect coverage changes

[MrAlias]

LGTM. I've added this to the SIG meeting agenda for tomorrow so we can review and close the issue.

[pellared]

This demonstrates that an option on histogram instruments for providing explicit bucket boundaries can be added in a backwards-compatible manner.

LGTM

[MrAlias]

Reviewed in the SIG, and we concluded this satisfies the PoC requirement. Closing.