[SLS-1260] Add functionname attribute to span metadata

[hghotra]

What does this PR do?

Add the functionname attribute to the Lambda span metadata.

Motivation

Look at problem 2 here: https://docs.google.com/document/d/1Jd40XCKibvhVntIiT02xjaZZlq3P2ohRHZ8VnKtu46Q/edit#

Testing Guidelines

Check the trace metadata tags to ensure the functionname attribute is present.

Additional Notes

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
• [ ] This PR collects user input/sensitive content into Datadog
• [ ] This PR passes the integration tests (ask a Datadog member to run the tests)

[codecov-commenter]

Codecov Report

Merging #89 (bdfb945) into main (c88b4f4) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
+ Coverage   84.93%   84.95%   +0.01%     
==========================================
  Files          13       13              
  Lines         770      771       +1     
==========================================
+ Hits          654      655       +1     
  Misses         91       91              
  Partials       25       25              

Impacted Files	Coverage Δ
internal/trace/listener.go	59.01% <100.00%> (+0.68%)	:arrow_up:

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