metricspi: Scrape metrics from MinIO backup storage

MinIO exposes metrics in Prometheus exposition format.  By default, it
requires an authentication token to access the metrics, but I was unable
to get this to work.  Fortunately, it can be configured to allow
anonymous access to the metrics, which is fine, in my opinion.

roles/minio/templates/minio.env.j2

@@ -2,3 +2,5 @@ MINIO_ROOT_USER={{ minio_root_user }}
 MINIO_ROOT_PASSWORD={{ minio_root_password }}
 
 MINIO_CONSOLE_ADDRESS={{ minio_console_address }}
+
+MINIO_PROMETHEUS_AUTH_TYPE=public