Add prometheus exporter support (develop)

readme-vars.yml

@@ -61,10 +61,14 @@ opt_param_env_vars:
   - {env_var: "SIDEKIQ_THREADS", env_value: "5", desc: "The number of threads for sidekiq to use. See [notes](https://docs.joinmastodon.org/admin/scaling/#sidekiq-threads)."}
   - {env_var: "DB_POOL", env_value: "5", desc: "The size of the DB connection pool, must be *at least* the same as `SIDEKIQ_THREADS`. See [notes](https://docs.joinmastodon.org/admin/scaling/#sidekiq-threads)."}
   - {env_var: "NO_CHOWN", env_value: "", desc: "Set to `true` to skip chown of /config on init. *READ THE APPLICATION NOTES BEFORE SETTING THIS*."}
+  - {env_var: "MASTODON_PROMETHEUS_EXPORTER_ENABLED", env_value: "", desc: "If set to `true`, Mastodon’s Ruby processes (web & Sidekiq) will enable the Prometheus instrumentation."}
 param_usage_include_ports: true
 param_ports:
   - {external_port: "80", internal_port: "80", port_desc: "Port for web frontend"}
   - {external_port: "443", internal_port: "443", port_desc: "Port for web frontend"}
+opt_param_usage_include_ports: true
+opt_param_ports:
+  - {external_port: "9394", internal_port: "9394", port_desc: "Port for Prometheus metrics"}
 param_usage_include_vols: true
 param_volumes:
   - {vol_path: "/config", vol_host_path: "/path/to/{{ project_name }}/config", desc: "Contains all relevant configuration files."}
@@ -166,6 +170,7 @@ init_diagram: |
   "mastodon:develop" <- Base Images
 # changelog
 changelogs:
+  - {date: "21.10.25:", desc: "Add prometheus exporter support."}
   - {date: "20.10.25:", desc: "Add vips-heif."}
   - {date: "06.06.25:", desc: "Rebase to Alpine 3.22, replace deprecated imagemagick with vips."}
   - {date: "26.08.24:", desc: "Rebase to Alpine 3.20, enable [Active Record Encryption](https://github.com/mastodon/mastodon/pull/29831/files). Existing users should update their nginx confs to avoid http2 deprecation warnings."}

root/etc/s6-overlay/s6-rc.d/svc-prom/notification-fd

@@ -0,0 +1 @@
+5

root/etc/s6-overlay/s6-rc.d/svc-prom/run

@@ -0,0 +1,10 @@
+#!/usr/bin/with-contenv bash
+# shellcheck shell=bash
+
+export RAILS_ENV=production
+export HOME=/config
+export PATH="${PATH}:/app/www/bin"
+
+exec \
+    s6-notifyoncheck -d -n 300 -w 1000 -c "nc -z localhost ${PROMETHEUS_EXPORTER_PORT:-9394}" \
+    cd /app/www s6-setuidgid abc /app/www/bin/prometheus_exporter

root/etc/s6-overlay/s6-rc.d/svc-prom/type

@@ -0,0 +1 @@
+longrun

root/init-hook

@@ -3,6 +3,10 @@
 # Disable the php-fpm service as the container doesn't use it
 rm /etc/s6-overlay/s6-rc.d/user/contents.d/svc-php-fpm
 
+if [[ ${MASTODON_PROMETHEUS_EXPORTER_ENABLED,,} != "true" ]]; then
+    rm /etc/s6-overlay/s6-rc.d/user/contents.d/svc-prom
+fi
+
 if [[ ${SIDEKIQ_ONLY,,} == "true" ]]; then
     rm /etc/s6-overlay/s6-rc.d/user/contents.d/svc-mastodon
     rm /etc/s6-overlay/s6-rc.d/user/contents.d/svc-nginx