[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

• [x] 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)