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

[bcoe]

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.