feat: expose system metrics (#4)

Use prom_ex to expose system metrics to prometheus and create dashboards.

Co-authored-by: Joao P Dubas <joao.dubas@gmail.com>
Reviewed-on: #4

Makefile

@@ -36,6 +36,10 @@ compose_up:  ## start containers for this service
 compose_test:  ## run tests in containers
 	@$(COMPOSE) run -e MIX_ENV=test --entrypoint make app test
 
+.PHONY: compose_ps
+compose_ps:  ## status of containers
+	@$(COMPOSE) ps
+
 .PHONY: help
 help:
 	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) | sort | awk 'BEGIN {FS = ":.*?## "}; {printf "\033[36m%-30s\033[0m %s\n", $$1, $$2}'