Allow describing metrics

lib/gitlab_exporter/prometheus.rb

@@ -12,9 +12,14 @@ module GitLab
       def initialize(include_timestamp: true)
         @metrics = Hash.new { |h, k| h[k] = [] }
         @quantiles = Hash.new { |h, k| h[k] = [] }
+        @metric_descriptions = {}
         @include_timestamp = include_timestamp
       end
 
+      def describe(name, description)
+        @metric_descriptions[name] = description
+      end
+
       def add(name, value, quantile = false, **labels)
         fail "value '#{value}' must be a number" unless value.is_a?(Numeric)
 
@@ -32,6 +37,8 @@ module GitLab
 
         buffer = ""
         @metrics.each do |name, measurements|
+          buffer << "# HELP #{name} #{@metric_descriptions[name]}\n" if @metric_descriptions[name]
+
           measurements.each do |measurement|
             buffer << name.to_s
             labels = (measurement[:labels] || {}).map { |label, value| "#{label}=\"#{value}\"" }.join(",")

spec/prometheus_metrics_spec.rb

@@ -23,4 +23,19 @@ describe GitLab::Exporter::PrometheusMetrics do
       subject.add("mymetric", "invalid", mylabel: "x", myotherlabel: "y").to_s
     }.to raise_error(RuntimeError)
   end
+
+  it "supports described metrics" do
+    time = Time.now
+
+    allow(Time).to receive(:now).and_return(time)
+
+    subject.describe("mymetric", "description")
+    subject.describe("missingmetric", "otherdescription")
+    subject.add("mymetric", 1.3, mylabel: "x", myotherlabel: "y")
+
+    expect(subject.to_s).to eq(<<~METRICS)
+      # HELP mymetric description
+      mymetric{mylabel="x",myotherlabel="y"} 1.3 #{(time.to_f * 1000).to_i}
+    METRICS
+  end
 end