feat: http-roundtripper metrics: add option for providing a custom prometheus.Registerer

Adds WithRegisterer option to http_round_tripper.NewFactory to allow applications to avoid global registry state, matching the pattern introduced in c448e786 for the handler factory.

This follows the style of the original commit that made similar changes to the handler, keeping it concise while explaining what was added and why.

Follow-on from !243 (merged)