Fix disabling of cold start tracing

[henrist]

What does this PR do?

Fixes so that cold start tracing can be disabled as documented.

The value from getEnvValue is a string, and unless this is a blank string it will always be truthy, even when the value is "false".

Use the similar pattern as used in the config section.

Motivation

I've tested out provisioned concurrency, but this makes tracing very inconvenient to use as it might generate traces that lasts for a very long time (so a 50 ms lambda might show a trace for 1h+). I wanted to disable cold start tracing to avoid those cases.

Ideally, maybe cold starts outside an invocation (as for provisioned concurrency), should not be included by default, but cold starts that took place during an invocation should be included.

Testing Guidelines

Additional Notes

Types of Changes

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

Check all that apply

• [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
• [ ] This PR passes the integration tests (ask a Datadog member to run the tests)

[codecov-commenter]

Codecov Report

Merging #370 (545e4b8) into main (76021dc) will not change coverage. The diff coverage is 100.00%.

:exclamation: Current head 545e4b8 differs from pull request most recent head 9a0dcd2. Consider uploading reports for the commit 9a0dcd2 to get more accurate results

@@           Coverage Diff           @@
##             main     #370   +/-   ##
=======================================
  Coverage   80.60%   80.60%           
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  Files          38       38           
  Lines        1903     1903           
  Branches      439      439           
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  Hits         1534     1534           
  Misses        313      313           
  Partials       56       56           

Impacted Files	Coverage Δ
src/index.ts	87.59% <100.00%> (ø)

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