Add support for upstream response duration metric

[Firehed]

This is blocked by upsteam support, tracked at influxdata/nginx-influxdb-module#3. It will also likely require a version bump in the ingress controller itself. Since it will be based on entirely new data, conditional logic will be required to handle the data not being present.

[Firehed]

Updated the title to more accurately reflect what this data would be. I had initially thought that the existing request_time (see #1) was the nginx-exclusive duration, but in fact it's the end-to-end request duration. That certainly makes this issue way less important, though still useful.