feat: Support DD_TRACE_OTEL_ENABLED env var

[astuyve]

What does this PR do?

Honors the DD_TRACE_OTEL_ENABLED env var so users can utilize the otel SDK in Lambda. Allows users to mix and match:

Motivation

Testing Guidelines

Additional Notes

Types of Changes

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

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• 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
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[joeyzhao2018]

LGTM