prometheus_exporter installed and initial docu written

Gemfile

@@ -144,3 +144,4 @@ end
 group :test, :development, :docker_development do
   gem 'cypress-on-rails', '~> 1.0'
 end
+gem 'prometheus_exporter'

Gemfile.lock

@@ -383,6 +383,8 @@ GEM
     progress_bar (1.3.3)
       highline (>= 1.6, < 3)
       options (~> 2.3.0)
+    prometheus_exporter (0.7.0)
+      webrick
     public_suffix (4.0.6)
     puma (4.3.7)
       nio4r (~> 2.0)
@@ -606,6 +608,7 @@ GEM
       rack-proxy (>= 0.6.1)
       railties (>= 5.2)
       semantic_range (>= 2.3.0)
+    webrick (1.7.0)
     websocket-driver (0.7.3)
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.5)
@@ -668,6 +671,7 @@ DEPENDENCIES
   pgreset
   premailer-rails
   progress_bar
+  prometheus_exporter
   puma (~> 4.1)
   rack
   rails (~> 6.1)

MONITORING.md

@@ -0,0 +1,30 @@
+# Monitoring
+
+## Getting started (development mode): Setting up prometheus & grafana
+
+0. Start prometheus_exporter in the mampf container
+`sudo docker-compose exec mampf prometheus_exporter -b 0.0.0.0`
+1. Setup prometheus in development
+
+```sh
+cd docker/development
+sudo docker run -d \
+-p 9090:9090 \
+--name prometheus -v $(pwd)/prometheus.yml:/etc/prometheus/prometheus.yml \
+prom/prometheus
+# Add to mampf dev network
+sudo docker network connect development_default prometheus
+```
+2. setup Grafana:
+
+```sh
+sudo docker run -d \                                      
+-p 2345:2345 \
+--name grafana \
+-e "GF_SERVER_HTTP_PORT=2345" \
+grafana/grafana
+```
+3. Now visit localhost:2345 and configure the datasource (`prometheus:9090`)
+4.  Setup the dashboard, interisting metrics:
+  - `rate(ruby_collector_sessions_total[5m])`
+  - `rate(ruby_http_requests_total[5m])`

config/initializers/prometheus_exporter.rb

@@ -0,0 +1,7 @@
+# in config/initializers/prometheus.rb
+if Rails.env != "test"
+    require 'prometheus_exporter/middleware'
+
+    # This reports stats per request like HTTP status and timings
+    Rails.application.middleware.unshift PrometheusExporter::Middleware
+  end