Add prometheus metrics endpoint.

Gemfile

@@ -17,6 +17,7 @@ gem "secure_headers"
 gem "heroku-env"
 gem "activesupport"
 gem "nokogiri"
+gem "prometheus-client", require: "prometheus/middleware/exporter"
 
 # dilbert feed
 gem "feedjira"

Gemfile.lock

@@ -48,6 +48,8 @@ GEM
       nokogiri
     powder (0.4.0)
       thor (>= 0.11.5)
+    prometheus-client (0.8.0)
+      quantile (~> 0.2.1)
     pry (0.12.2)
       coderay (~> 1.1.0)
       method_source (~> 0.9.0)
@@ -56,6 +58,7 @@ GEM
       slop (~> 3.0)
     public_suffix (3.0.3)
     puma (3.12.0)
+    quantile (0.2.1)
     rack (2.0.6)
     rack-protection (2.0.4)
       rack
@@ -102,6 +105,7 @@ DEPENDENCIES
   nokogiri
   opengraph_parser
   powder
+  prometheus-client
   pry-remote
   puma
   rack

app/twitter.rb

@@ -9,6 +9,14 @@ class Twitter < HTTP
     "Authorization" => "Bearer #{ENV["TWITTER_ACCESS_TOKEN"]}",
   }
   ERROR_CLASS = TwitterError
+
+  def self.get(*args, &block)
+    response = super(*args, &block)
+    if response.headers.key?("x-rate-limit-remaining")
+      $metrics[:ratelimit].set({ service: "twitter" }, response.headers["x-rate-limit-remaining"][0].to_i)
+    end
+    return response
+  end
 end
 
 error TwitterError do |e|

config/application.rb

@@ -12,6 +12,8 @@
   use Rack::Deflater
   use Rack::SslEnforcer, only_hosts: (ENV["SSL_ENFORCER_HOST"] || /\.herokuapp\.com$/)
   use SecureHeaders::Middleware
+  use Prometheus::Middleware::Exporter
+
   set :protection, :except => [:frame_options] # Disable things that secure_headers handles
   set :erb, trim: "-"
   # Look up Rack::Mime::MIME_TYPES to see rack defaults

config/initializers/40-prometheus.rb

@@ -0,0 +1,7 @@
+$prometheus = Prometheus::Client.registry
+
+$metrics = {
+  ratelimit: $prometheus.gauge(:ratelimit, docstring: "Remaining ratelimit for external services.", labels: [:service]),
+  requests: $prometheus.counter(:requests, docstring: "Number of requests made to external services.", labels: [:service, :response_code]),
+  urls: $prometheus.counter(:urls, docstring: "Number of URLs resolved."),
+}

docker-compose.yml

@@ -1,3 +1,5 @@
+# Grafana is available at http://localhost:3001/. Login with admin/admin. Configure a Prometheus data source to pull metrics from http://prometheus:9090.
+
 version: "3"
 services:
   web:
@@ -10,13 +12,34 @@ services:
     ports:
       - "3000:80"
     networks:
-      - rssbox
+      - backend
+      - monitoring
     depends_on:
       - redis
+
   redis:
     image: redis
     networks:
-      - rssbox
+      - backend
+
+  prometheus:
+    image: prom/prometheus
+    ports:
+      - "9090:9090"
+    volumes:
+      - ${PWD}/prometheus.yml:/etc/prometheus/prometheus.yml
+    networks:
+      - monitoring
+
+  grafana:
+    image: grafana/grafana
+    ports:
+      - "3001:3000"
+    networks:
+      - monitoring
+    depends_on:
+      - prometheus
 
 networks:
-  rssbox:
+  backend:
+  monitoring:

lib/http.rb

@@ -36,6 +36,7 @@ def self.get(url, options={headers: nil, query: nil})
       headers.merge!(self::HEADERS) if defined?(self::HEADERS) && relative_url
       headers.merge!(options[:headers]) if options[:headers]
       response = http.request_get(uri.request_uri, headers)
+      $metrics[:requests].increment({ service: self.to_s.downcase, response_code: response.code })
       return HTTPResponse.new(response, uri.to_s)
     end
   rescue Net::OpenTimeout, Net::ReadTimeout, SocketError, Errno::ECONNREFUSED, Errno::ECONNRESET, OpenSSL::SSL::SSLError, Zlib::BufError, EOFError

lib/url.rb

@@ -131,5 +131,7 @@ def self.resolve_url(url)
     dest = "" if url == dest
     @@cache[url] = dest
     $redis.set("url:#{url}", dest)
+
+    $metrics[:urls].increment
   end
 end

prometheus.yml

@@ -0,0 +1,13 @@
+global:
+  scrape_interval: "15s"
+  scrape_timeout: "10s"
+  evaluation_interval: "15s"
+
+scrape_configs:
+- job_name: "rssbox"
+  metrics_path: "/metrics"
+  scheme: "http"
+  static_configs:
+  - targets:
+    - "web:80"
+    # - "host.docker.internal:3000"