closes #3104 - add support for exposing prometheus metrics

docs/additional-features/prometheus-metrics.md

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+# Prometheus Metrics
+
+NetBox supports optionally exposing native Prometheus metrics from the application. [Prometheus](https://prometheus.io/) is a popular time series metric platform used for monitoring.
+
+NetBox exposes metrics at the `/metrics` HTTP endpoint, e.g. `https://netbox.local/metrics`. Metric exposition can be toggled with the `METRICS_ENABLED` configuration setting. Metrics are exposed by default.
+
+## Metric Types
+
+NetBox makes use of the [django-prometheus](https://github.com/korfuri/django-prometheus) library to export a number of different types of metrics, including:
+
+- Per model insert, update, and delete counters
+- Per view request counters
+- Per view request latency histograms
+- Request body size histograms
+- Response body size histograms
+- Response code counters
+- Database connection, execution, and error counters
+- Cache hit, miss, and invalidation counters
+- Django middleware latency histograms
+- Other Django related metadata metrics
+
+For the exhaustive list of exposed metrics, visit the `/metrics` endpoint on your NetBox instance.