r/frigate-exporter: Deploy Prometheus exporter

Frigate exports useful statistics natively, but in a custom JSON format.
There is a [feature request][0] to add support for Prometheus format,
but it's mostly being ignored.  A community member has created a
standalone process that converts the JSON format into Prometheus format,
though, which we can use.

[0]: https://github.com/blakeblackshear/frigate/issues/2266

frigate.yml

@@ -6,3 +6,5 @@
     tags: frigate
   - role: frigate-caddy
     tags: frigate-caddy
+  - role: frigate-exporter
+    tags: frigate-exporter