profiler: explain how to profile third-party Go services

[nsrip-dd]

What does this PR do? What is the motivation?

We have an Agent integration for collecting profiles from Go services through a Go standard library debug HTTP endpoint. Document its existence. Customers can use this integration to profile Go services for which they don't control the source code and can't use our client library. For example, Kubernetes cluster administrators can use this to profile the Kubernetes API server. We still prefer that customers use our client library if they can, to get access to the full range of features.

Merge instructions

• [ ] Please merge after reviewing