Prototype of OTEPs 168 and 170 Probability Sampling

[jmacd]

See open-telemetry/oteps#168 and open-telemetry/oteps#170.

This is a DRAFT for reviewing the associated OTEPs, not for merging.

[codecov]

Codecov Report

Merging #2177 (ec930a6) into main (b45c9d3) will increase coverage by 0.2%.
The diff coverage is 89.8%.

@@           Coverage Diff           @@
##            main   #2177     +/-   ##
=======================================
+ Coverage   72.4%   72.6%   +0.2%     
=======================================
  Files        176     176             
  Lines      12095   12354    +259     
=======================================
+ Hits        8762    8981    +219     
- Misses      3097    3127     +30     
- Partials     236     246     +10     

Impacted Files	Coverage Δ
sdk/trace/sampling.go	90.8% <89.3%> (-9.2%)	⬇️
sdk/trace/attributesmap.go	93.7% <100.0%> (+1.2%)	⬆️
sdk/metric/refcount_mapped.go	80.0% <0.0%> (-20.0%)	⬇️
sdk/metric/sdk.go	79.5% <0.0%> (-1.5%)	⬇️
sdk/trace/batch_span_processor.go	83.8% <0.0%> (-0.6%)	⬇️