fix: reading sampling priority will error when not available

[duncanista]

What does this PR do?

Fixes a bug in which reading sampling priority will cause an error, because it was undefined. Previously, before adding W3C, we'd just set always AUTO_KEEP, but now we decided on getting whatever the trace context had.

Now, if not available, we'll go back to using AUTO_KEEP, while respecting whichever was there before if set.

Motivation

#482

Testing Guidelines

Added unit tests.

Additional Notes

Introduced in: f05c600#diff-8367049249917f97a7d4b0d7aa3355ac631ce80bd38d35f0f04c9288c603b76a

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
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[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (a481762) 82.12% compared to head (226cee8) 82.13%.

❗ Current head 226cee8 differs from pull request most recent head bf0b1ae. Consider uploading reports for the commit bf0b1ae to get more accurate results

Files	Patch %	Lines
src/trace/span-context-wrapper.ts	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #483   +/-   ##
=======================================
  Coverage   82.12%   82.13%           
=======================================
  Files          54       54           
  Lines        2188     2189    +1     
  Branches      510      510           
=======================================
+ Hits         1797     1798    +1     
  Misses        329      329           
  Partials       62       62           

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[astuyve]

Good use of null coalescence here, 0 || would be falsey and could cause us to keep everything and break sampling (if priority is an int)