[CONFIGURATION] Apply default sampler when none is specified

[RaviTriv]

Fixes #4034

Changes

PR to apply default sampler when one is not set in the trace provider configuration. Updated the parser to apply ParentBasedSamplerConfiguration with AlwaysOnSamplerConfiguration as per the spec when no sampler is specified.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.90%. Comparing base (56848f9) to head (482af5d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4170      +/-   ##
==========================================
+ Coverage   82.88%   82.90%   +0.03%     
==========================================
  Files         405      405              
  Lines       17287    17292       +5     
==========================================
+ Hits        14326    14334       +8     
+ Misses       2961     2958       -3     

Files with missing lines	Coverage Δ
sdk/src/configuration/configuration_parser.cc	77.65% <100.00%> (+0.09%)	⬆️

... and 2 files with indirect coverage changes

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[lalitb]

I think there is one remaining default-sampler edge case. This PR handles the case where sampler is omitted entirely, but an explicit:

sampler:
  parent_based:

still leaves ParentBasedSamplerConfiguration::root as nullptr.

The config spec says ParentBasedSampler.root defaults to always_on when omitted. Since SdkBuilder::CreateParentBasedSampler() later calls CreateSampler(model->root), should we also apply the always_on default when parent_based.root is omitted?

[RaviTriv]

I think there is one remaining default-sampler edge case. This PR handles the case where sampler is omitted entirely, but an explicit:

sampler:
  parent_based:

still leaves ParentBasedSamplerConfiguration::root as nullptr.

The config spec says ParentBasedSampler.root defaults to always_on when omitted. Since SdkBuilder::CreateParentBasedSampler() later calls CreateSampler(model->root), should we also apply the always_on default when parent_based.root is omitted?

Good catch! Updated the parser to apply the always_on default when parent_based.root is omitted.

[marcalff]

LGTM, see a minor comment on the tests.

[marcalff]

LGTM, thanks for the fix