Add Gauge#set_to_current_time

I was porting some code from Python to Ruby today, and when I went to
call this method, I realised we don't have it!

My use-case is to track the last time some code successfully ran.

I'm open to discussing the naming, but I've defaulted to the name used
in `client_python` because it seems good to me.

Signed-off-by: Chris Sinjakli <chris@sinjakli.co.uk>

lib/prometheus/client/gauge.rb

@@ -20,6 +20,10 @@ def set(value, labels: {})
         @store.set(labels: label_set_for(labels), val: value)
       end
 
+      def set_to_current_time(labels: {})
+        @store.set(labels: label_set_for(labels), val: Time.now.to_f)
+      end
+
       # Increments Gauge value by 1 or adds the given value to the Gauge.
       # (The value can be negative, resulting in a decrease of the Gauge.)
       def increment(by: 1, labels: {})

prometheus-client.gemspec

@@ -17,4 +17,5 @@ Gem::Specification.new do |s|
 
   s.add_development_dependency 'benchmark-ips'
   s.add_development_dependency 'concurrent-ruby'
+  s.add_development_dependency 'timecop'
 end

spec/prometheus/client/gauge_spec.rb

@@ -48,6 +48,30 @@
     end
   end
 
+  describe '#set_to_current_time' do
+    it 'it sets the gauge to the current Unix epoch time' do
+      Timecop.freeze(Time.at(12345.1)) do
+        expect do
+          gauge.set_to_current_time
+        end.to change { gauge.get }.from(0).to(12345.1)
+      end
+    end
+
+    context "with a an expected label set" do
+      let(:expected_labels) { [:test] }
+
+      it 'sets a metric value for a given label set' do
+        Timecop.freeze(Time.at(12345.1)) do
+          expect do
+            expect do
+              gauge.set_to_current_time(labels: { test: 'value' })
+            end.to change { gauge.get(labels: { test: 'value' }) }.from(0).to(12345.1)
+          end.to_not change { gauge.get(labels: { test: 'other' }) }
+        end
+      end
+    end
+  end
+
   describe '#increment' do
     before do
       gauge.set(0, labels: RSpec.current_example.metadata[:labels] || {})

spec/spec_helper.rb

@@ -1,6 +1,7 @@
 # encoding: UTF-8
 
 require 'simplecov'
+require 'timecop'
 
 RSpec.configure do |c|
   c.warnings = true
@@ -9,3 +10,5 @@
 SimpleCov.formatter = SimpleCov::Formatter::HTMLFormatter
 
 SimpleCov.start
+
+Timecop.safe_mode = true