Add counter warmup option

bin/prometheus_exporter

@@ -71,6 +71,10 @@ def run
     opt.on('--logger-path PATH', String, '(optional) Path to file for logger output. Defaults to STDERR') do |o|
       options[:logger_path] = o
     end
+
+    opt.on('--counter-warmup', "Automate initialization of counter values to 0") do |o|
+      options[:counter_warmup] = true
+    end
   end.parse!
 
   logger = Logger.new(options[:logger_path])

lib/prometheus_exporter/metric/counter.rb

@@ -2,6 +2,17 @@
 
 module PrometheusExporter::Metric
   class Counter < Base
+
+    @counter_warmup_enabled = nil if !defined?(@counter_warmup_enabled)
+
+    def self.counter_warmup=(enabled)
+      @counter_warmup_enabled = enabled
+    end
+
+    def self.counter_warmup
+      !!@counter_warmup_enabled
+    end
+
     attr_reader :data
 
     def initialize(name, help)
@@ -15,10 +26,12 @@ def type
 
     def reset!
       @data = {}
+      @counter_warmup = {}
     end
 
     def metric_text
-      @data.map do |labels, value|
+      @data.keys.map do |labels|
+        value = warmup_counter_value(labels)
         "#{prefix(@name)}#{labels_text(labels)} #{value}"
       end.join("\n")
     end
@@ -28,26 +41,43 @@ def to_h
     end
 
     def remove(labels)
+      @counter_warmup.delete(labels)
       @data.delete(labels)
     end
 
     def observe(increment = 1, labels = {})
+      warmup_counter(labels)
       @data[labels] ||= 0
       @data[labels] += increment
     end
 
     def increment(labels = {}, value = 1)
+      warmup_counter(labels)
       @data[labels] ||= 0
       @data[labels] += value
     end
 
     def decrement(labels = {}, value = 1)
+      warmup_counter(labels)
       @data[labels] ||= 0
       @data[labels] -= value
     end
 
     def reset(labels = {}, value = 0)
+      warmup_counter(labels)
       @data[labels] = value
     end
+
+    private
+
+    def warmup_counter(labels)
+      if Counter.counter_warmup && [email protected]_key?(labels)
+        @counter_warmup[labels] = 0
+      end
+    end
+
+    def warmup_counter_value(labels)
+      @counter_warmup.delete(labels) || @data[labels]
+    end
   end
 end

lib/prometheus_exporter/server/process_collector.rb

@@ -28,6 +28,11 @@ def initialize
       @process_metrics.filter = -> (new_metric, old_metric) do
         new_metric["pid"] == old_metric["pid"] && new_metric["hostname"] == old_metric["hostname"]
       end
+      @counter_metrics = {}
+      PROCESS_COUNTERS.each do |k, help|
+        k = k.to_s
+        @counter_metrics[k] = PrometheusExporter::Metric::Counter.new(k, help)
+      end
     end
 
     def type
@@ -43,18 +48,18 @@ def metrics
         metric_key = (m["metric_labels"] || {}).merge("pid" => m["pid"], "hostname" => m["hostname"])
         metric_key.merge!(m["custom_labels"]) if m["custom_labels"]
 
-        PROCESS_GAUGES.map do |k, help|
+        PROCESS_GAUGES.each do |k, help|
           k = k.to_s
           if v = m[k]
             g = metrics[k] ||= PrometheusExporter::Metric::Gauge.new(k, help)
             g.observe(v, metric_key)
           end
         end
 
-        PROCESS_COUNTERS.map do |k, help|
+        PROCESS_COUNTERS.each do |k, help|
           k = k.to_s
           if v = m[k]
-            c = metrics[k] ||= PrometheusExporter::Metric::Counter.new(k, help)
+            c = metrics[k] ||= @counter_metrics[k]
             c.observe(v, metric_key)
           end
         end

lib/prometheus_exporter/server/runner.rb

@@ -17,6 +17,7 @@ def initialize(options = {})
       @auth = nil
       @realm = nil
       @histogram = nil
+      @counter_warmup = nil
 
       options.each do |k, v|
         send("#{k}=", v) if self.class.method_defined?("#{k}=")
@@ -26,6 +27,7 @@ def initialize(options = {})
     def start
       PrometheusExporter::Metric::Base.default_prefix = prefix
       PrometheusExporter::Metric::Base.default_labels = label
+      PrometheusExporter::Metric::Counter.counter_warmup = counter_warmup
 
       if histogram
         PrometheusExporter::Metric::Base.default_aggregation = PrometheusExporter::Metric::Histogram
@@ -54,7 +56,7 @@ def start
     end
 
     attr_accessor :unicorn_listen_address, :unicorn_pid_file
-    attr_writer :prefix, :port, :bind, :collector_class, :type_collectors, :timeout, :verbose, :server_class, :label, :auth, :realm, :histogram
+    attr_writer :prefix, :port, :bind, :collector_class, :type_collectors, :timeout, :verbose, :server_class, :label, :auth, :realm, :histogram, :counter_warmup
 
     def auth
       @auth || nil
@@ -109,6 +111,10 @@ def histogram
       @histogram || false
     end
 
+    def counter_warmup
+      @counter_warmup || false
+    end
+
     private
 
     def register_type_collectors

test/metric/counter_test.rb

@@ -143,5 +143,36 @@ module PrometheusExporter::Metric
 
       assert_equal(counter.to_h, { sam: "ham" } => 5, { foo: "bar" } => 10)
     end
+
+    describe "with counter warmup enabled" do
+      before do
+        Counter.counter_warmup = true
+      end
+
+      after do
+        Counter.counter_warmup = false
+      end
+
+      it "returns 0 on 1st read of text, actual value on 2nd read" do
+        counter.observe(5, sam: "ham")
+        counter.observe(10, foo: "bar")
+
+        text = <<~TEXT
+          # HELP a_counter my amazing counter
+          # TYPE a_counter counter
+          a_counter{sam="ham"} 0
+          a_counter{foo="bar"} 0
+        TEXT
+        assert_equal(counter.to_prometheus_text, text)
+
+        text = <<~TEXT
+          # HELP a_counter my amazing counter
+          # TYPE a_counter counter
+          a_counter{sam="ham"} 5
+          a_counter{foo="bar"} 10
+        TEXT
+        assert_equal(counter.to_prometheus_text, text)
+      end
+    end
   end
 end

test/server/runner_test.rb

@@ -3,6 +3,8 @@
 require_relative '../test_helper'
 require 'prometheus_exporter/server'
 
+require 'ostruct'
+
 class PrometheusRunnerTest < Minitest::Test
   class MockerWebServer < OpenStruct
     def start
@@ -70,7 +72,8 @@ def test_runner_custom_options
       label: { environment: 'integration' },
       auth: 'my_htpasswd_file',
       realm: 'test realm',
-      histogram: true
+      histogram: true,
+      counter_warmup: true
     )
 
     assert_equal(runner.prefix, 'new_')
@@ -83,6 +86,7 @@ def test_runner_custom_options
     assert_equal(runner.auth, 'my_htpasswd_file')
     assert_equal(runner.realm, 'test realm')
     assert_equal(runner.histogram, true)
+    assert_equal(runner.counter_warmup, true)
 
     reset_base_metric_label
   end