add DD_TRACE_OTEL_ENABLED support

[joeyzhao2018]

What does this PR do?

To better support Open-telemetry syntax, initialize the tracer provider if DD_TRACE_OTEL_ENABLED is enabled.

With this change, users can directly import go.opentelemetry.io/otel and use

	tracer := otel.Tracer("customer-code") //otel
	_, span := tracer.Start(ctx, "hello-span")  //otel

Motivation

This is because we do initialize the tracer with a set of options in this library.

Testing Guidelines

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