Start tracer while initializing trace lambda listener

[shreyamalpani]

dd-trace-go PR

DataDog/dd-trace-go#4371

What does this PR do?

Starts the tracer while initializing the lambda listener in instead of on handler started

Motivation

SLES-2658 Fast initializing functions hitting timeouts

Testing Guidelines

Tested 50k invocations with a simple Go Lambda handler

Before:

After:

Additional Notes

Types of changes

• Bug fix
• New feature
• Breaking change
• 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