Automatically initialize time series without labels

According to [Prometheus Best Practices](https://prometheus.io/docs/practices/instrumentation/#avoid-missing-metrics),
client libraries are expected to automatically export a 0 value when declaring
a metric that has no labels.

We missed this so far in the Ruby client, this PR rectifies that.

NOTE: This can be considered a breaking change. On the one hand, it's a bug fix,
but on the other, it will make many time series materialize when scraping apps
that use the client, that previously wouldn't have.

Depending on how many label-less metrics the app is declaring, this may have a
significant impact, so we should probably cut a new major version and warn
about this in the Release Notes.

lib/prometheus/client/metric.rb

@@ -40,6 +40,8 @@ def initialize(name,
           metric_type: type,
           metric_settings: store_settings
         )
+
+        init_label_set({}) if labels.empty?
       end
 
       # Returns the value for the given label set

spec/prometheus/client/counter_spec.rb

@@ -104,14 +104,22 @@
   end
 
   describe '#init_label_set' do
-    let(:expected_labels) { [:test] }
+    context "with labels" do
+      let(:expected_labels) { [:test] }
 
-    it 'initializes the metric for a given label set' do
-      expect(counter.values).to eql({})
+      it 'initializes the metric for a given label set' do
+        expect(counter.values).to eql({})
 
-      counter.init_label_set(test: 'value')
+        counter.init_label_set(test: 'value')
 
-      expect(counter.values).to eql({test: 'value'} => 0.0)
+        expect(counter.values).to eql({test: 'value'} => 0.0)
+      end
+    end
+
+    context "without labels" do
+      it 'automatically initializes the metric' do
+        expect(counter.values).to eql({} => 0.0)
+      end
     end
   end
 end

spec/prometheus/client/gauge_spec.rb

@@ -186,14 +186,22 @@
   end
 
   describe '#init_label_set' do
-    let(:expected_labels) { [:test] }
+    context "with labels" do
+      let(:expected_labels) { [:test] }
 
-    it 'initializes the metric for a given label set' do
-      expect(gauge.values).to eql({})
+      it 'initializes the metric for a given label set' do
+        expect(gauge.values).to eql({})
 
-      gauge.init_label_set(test: 'value')
+        gauge.init_label_set(test: 'value')
 
-      expect(gauge.values).to eql({test: 'value'} => 0.0)
+        expect(gauge.values).to eql({test: 'value'} => 0.0)
+      end
+    end
+
+    context "without labels" do
+      it 'automatically initializes the metric' do
+        expect(gauge.values).to eql({} => 0.0)
+      end
     end
   end
 end

spec/prometheus/client/histogram_spec.rb

@@ -165,18 +165,28 @@
   end
 
   describe '#init_label_set' do
-    let(:expected_labels) { [:status] }
+    context "with labels" do
+      let(:expected_labels) { [:status] }
 
-    it 'initializes the metric for a given label set' do
-      expect(histogram.values).to eql({})
+      it 'initializes the metric for a given label set' do
+        expect(histogram.values).to eql({})
 
-      histogram.init_label_set(status: 'bar')
-      histogram.init_label_set(status: 'foo')
+        histogram.init_label_set(status: 'bar')
+        histogram.init_label_set(status: 'foo')
 
-      expect(histogram.values).to eql(
-        { status: 'bar' } => { "2.5" => 0.0, "5" => 0.0, "10" => 0.0, "+Inf" => 0.0, "sum" => 0.0 },
-        { status: 'foo' } => { "2.5" => 0.0, "5" => 0.0, "10" => 0.0, "+Inf" => 0.0, "sum" => 0.0 },
-      )
+        expect(histogram.values).to eql(
+          { status: 'bar' } => { "2.5" => 0.0, "5" => 0.0, "10" => 0.0, "+Inf" => 0.0, "sum" => 0.0 },
+          { status: 'foo' } => { "2.5" => 0.0, "5" => 0.0, "10" => 0.0, "+Inf" => 0.0, "sum" => 0.0 },
+        )
+      end
+    end
+
+    context "without labels" do
+      it 'automatically initializes the metric' do
+        expect(histogram.values).to eql(
+          {} => { "2.5" => 0.0, "5" => 0.0, "10" => 0.0, "+Inf" => 0.0, "sum" => 0.0 },
+        )
+      end
     end
   end
 end

spec/prometheus/client/summary_spec.rb

@@ -128,18 +128,28 @@
   end
 
   describe '#init_label_set' do
-    let(:expected_labels) { [:status] }
+    context "with labels" do
+      let(:expected_labels) { [:status] }
 
-    it 'initializes the metric for a given label set' do
-      expect(summary.values).to eql({})
+      it 'initializes the metric for a given label set' do
+        expect(summary.values).to eql({})
 
-      summary.init_label_set(status: 'bar')
-      summary.init_label_set(status: 'foo')
+        summary.init_label_set(status: 'bar')
+        summary.init_label_set(status: 'foo')
 
-      expect(summary.values).to eql(
-        { status: 'bar' } => { "count" => 0.0, "sum" => 0.0 },
-        { status: 'foo' } => { "count" => 0.0, "sum" => 0.0 },
-      )
+        expect(summary.values).to eql(
+          { status: 'bar' } => { "count" => 0.0, "sum" => 0.0 },
+          { status: 'foo' } => { "count" => 0.0, "sum" => 0.0 },
+        )
+      end
+    end
+
+    context "without labels" do
+      it 'automatically initializes the metric' do
+        expect(summary.values).to eql(
+          {} => { "count" => 0.0, "sum" => 0.0 },
+        )
+      end
     end
   end
 end