Fix sampling rate

[nhulston]

What does this PR do?

Fixes sampling rates not being respected.

When a sampling priority is set on a span but not in the context, we should still propagate it to the Lambda extension. Previously the sampling priority was not being sent to the extension in this case, causing sampling rates to be incorrect.

Motivation

Sampling rates didn't work before, leading to 100% of traces being sampled regardless of the DD_TRACE_SAMPLING_RULES or DD_TRACE_SAMPLE_RATE environment variables.

Testing Guidelines

Manual testing. Before changes, we saw every trace was being sampled. Now, we see the correct number of traces being sampled depending on the sample rate environment variable.

I also added unit tests in extension_test.go.

Additional Notes

Types of changes

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

Checklist

• 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
• This PR collects user input/sensitive content into Datadog

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.43%. Comparing base (39700b5) to head (4915d91).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #180      +/-   ##
==========================================
+ Coverage   80.55%   81.43%   +0.87%     
==========================================
  Files          13       13              
  Lines         905      937      +32     
==========================================
+ Hits          729      763      +34     
+ Misses        145      142       -3     
- Partials       31       32       +1     

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[purple4reina]

Great work on this!