measure request duration in httpclient middleware

Created by: uwedeportivo

Turns out we already have http client middleware to meter requests. It currently has a counter vector. This PR adds a duration histogram for request latency. This middleware is currently only used to meter http requests to external services. I'm planning in future PRs to use the same middleware for the internal http requests between nodes in a sourcegraph instance (like frontend <-> repo-updater for example).