Change default sampler to ParentOrElse(AlwaysOn)

[MitchellDumovic]

Resolves #1445 and implements the changes outlined in open-telemetry/opentelemetry-specification#728

[codecov]

Codecov Report

Merging #1474 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #1474   +/-   ##
=========================================
  Coverage     92.50%   92.50%           
  Complexity      940      940           
=========================================
  Files           117      117           
  Lines          3351     3351           
  Branches        270      270           
=========================================
  Hits           3100     3100           
  Misses          169      169           
  Partials         82       82           

Impacted Files	Coverage Δ	Complexity Δ
...main/java/io/opentelemetry/sdk/trace/Samplers.java	90.90% <100.00%> (ø)	13.00 <0.00> (ø)
...io/opentelemetry/sdk/trace/config/TraceConfig.java	100.00% <100.00%> (ø)	4.00 <1.00> (ø)

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[bogdandrutu]

Would wait for the specs PR to be merged

[thisthat]

Spec issue is now merged :) open-telemetry/opentelemetry-specification#750 (comment)