Include request / response headers as attributes on `http.client` spans

### Problem Statement

It would be valuable for answering certain types of performance questions to be able to see request and response headers on an `http.client` call.

As an example, `age` and `cf-cache-status` can help you determine whether a request was served from cache.

### Solution Brainstorm

Allow list certain headers, such as `age`, that we're confident are unlikely to include PII. Include these headers as attributes.