New port: net-mgmt/nvidia_gpu_prometheus_exporter: NVIDIA GPU Prometheus exporter

This is a Prometheus Exporter for exporting NVIDIA GPU metrics. It uses the
Go bindings for NVIDIA Management Library (NVML) which is a C-based API
that can be used for monitoring NVIDIA GPU devices. Unlike some other
similar exporters, it does not call the nvidia-smi binary.

WWW: https://github.com/mindprince/nvidia_gpu_prometheus_exporter

Reviewed by: mat
Approved by: mat (mentor)
Sponsored by: BALLY WULFF Games & Entertainment GmbH
Differential Revision: https://reviews.freebsd.org/D19002