Set default sampling priority to sampler-keep

[agocs]

What does this PR do?

Fixes #74. If datadog-lambda-go comes across a trace that does not have sampling-priority set, it should set a default priority (I think Tian decided on sampler-keep, but I will confirm.) This matches the behavior of other datadog-lambda-* libraries.

Motivation

Testing Guidelines

Additional Notes

Types of changes

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

Checklist

• [x] 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

Merging #88 (5d5d9a2) into main (d6fcbc3) will increase coverage by 0.25%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
+ Coverage   84.93%   85.19%   +0.25%     
==========================================
  Files          13       13              
  Lines         770      770              
==========================================
+ Hits          654      656       +2     
+ Misses         91       90       -1     
+ Partials       25       24       -1     

Impacted Files	Coverage Δ
internal/trace/context.go	78.12% <100.00%> (+1.56%)	:arrow_up:

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