add a new internal tracer to be used by auto-instrumentation

[rochdev]

What does this PR do?

Add a new internal tracer to be used by auto-instrumentation.

Motivation

The API that is exposed by dd-trace does a lot of conversion on the data to be as easy to use as possible for users. This has performance implications and we don't need these conversions internally. Using an internal data format that is closer to what will be sent on the wire makes more sense and is a lot faster and more efficient.