Large content in info response impacts performance a lot

[trallnag]

Raised by @bbeattie-phxlabs.

Collection of body content doesn't matter much when the response size is under a few megs, but when a response is a FileResponse beyond a couple hundred megs, it really starts to matter.

Considering response.body isn't used in any metrics in metrics.py, I'd suggest removing its collection altogether.

Hmm... I see this was added in for #203. I wonder if the better approach here would be to customize the send_wrapper based on a PrometheusInstrumentatorMiddleware instance variable, allowing this body aggregation for those that choose to enable it, or disabled for those that choose to disable it, depending on the approach.

[trallnag]

Released in https://github.com/trallnag/prometheus-fastapi-instrumentator/releases/tag/v6.0.0