Today there is no way for an extension writer to access the individual requests that are sent by the router (or any other HTTP client). We should extend the API to support adding upstream HTTP filters: same interface as the existing HTTP filters, but they apply to outbound requests.

Originally I envisioned these to be on the router (similar to how the upstream logs are defined there), but perhaps it makes the most sense to put them on the cluster level so that they can apply to any HTTP call made through Envoy? Thinking about calls made through the AsyncClient