feat: enable universal instrumentation by default

[duncanista]

What does this PR do?

Enabled universal instrumentation and tracing by default.

Honors environment variable if set.

Motivation

Multiple tickets of customers not understanding why they need to have another env var to get inferred spans ready.

Testing Guidelines

• Updated unit tests to respect previous behavior: tracing and universal instrumentation disabled.

Additional Notes

Types of changes

• [ ] Bug fix
• [ ] New feature
• [ ] Breaking change
• [x] 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

Attention: 22 lines in your changes are missing coverage. Please review.

Comparison is base (e8e55b8) 83.37% compared to head (0965f08) 82.12%. Report is 6 commits behind head on main.

:exclamation: Current head 0965f08 differs from pull request most recent head e964357. Consider uploading reports for the commit e964357 to get more accurate results

Files	Patch %	Lines
ddlambda.go	36.66%	18 Missing and 1 partial :warning:
internal/trace/listener.go	50.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #151 +/- ## ========================================== - Coverage 83.37% 82.12% -1.25% ========================================== Files 13 13 Lines 848 884 +36 ========================================== + Hits 707 726 +19 - Misses 112 129 +17 Partials 29 29 ```

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