capture body

This is about adding something similar to the classic APM agent's [`captureBody`](https://www.elastic.co/docs/reference/apm/agents/nodejs/configuration#capture-body).

There is a lot to work on here:
- `captureBody` had various settings: `all`, `errors`, `transactions`. To what degree can we or do we want to replicate those.
- Truncation for large bodies?
- Limits on which content-types will be included
- Will/should/must there be sanitization on capture bodies?
- What web frameworks/libs have instrumentation that *support* body capture at all?
- Be clear that streaming requests/responses generally cannot be captured without difficult impact/complexity in the instrumentations -- generally not accepted upstream. I.e. streaming requests are out.
- Perhaps docs to suggest structured logging in user code as alternative.
- Centrally configurable? The intention is "yes".
- Just for HTTP? Or also for messaging?
- Just for *incoming* (vs outgoing) requests?
- Just for *request* body (vs response body)?