Add testing guide.

examples/testing/app.rb

@@ -0,0 +1,16 @@
+require 'metrics/provider'
+
+class MyApplication
+	def work
+		# ...
+	end
+
+	Metrics::Provider(self) do
+		WORK_METRIC = Metrics.metric('my_application.work.count', :counter, description: 'Work counter')
+
+		def work
+			WORK_METRIC.emit(1)
+			super
+		end
+	end
+end

examples/testing/test.rb

@@ -0,0 +1,12 @@
+#!/usr/bin/env sus
+
+ENV['METRICS_BACKEND'] ||= 'metrics/backend/test'
+
+require_relative 'app'
+
+describe MyApplication do
+	it 'should emit metrics' do
+		expect(MyApplication::WORK_METRIC).to receive(:emit).with(1)
+		MyApplication.new.work
+	end
+end

guides/links.yaml

@@ -1,4 +1,6 @@
 getting-started:
   order: 1
 capture:
-  order: 2
+  order: 2
+testing:
+  order: 3

guides/testing/readme.md

@@ -0,0 +1,43 @@
+# Testing
+
+This guide explains how to write assertions in your test suite to validate `metrics` are being emitted correctly.
+
+## Application Code
+
+In your application code, you should emit metrics, e.g.
+
+```ruby
+require 'metrics/provider'
+
+class MyApplication
+	def work
+		# ...
+	end
+
+	Metrics::Provider(self) do
+		WORK_METRIC = Metrics.metric('my_application.work.count', :counter, description: 'Work counter')
+
+		def work
+			WORK_METRIC.emit(1)
+			super
+		end
+	end
+end
+```
+
+## Test Code
+
+In your test code, you should assert that the metrics are being emitted correctly, e.g.
+
+```ruby
+ENV['METRICS_BACKEND'] ||= 'metrics/backend/test'
+
+require_relative 'app'
+
+describe MyApplication do
+	it 'should emit metrics' do
+		expect(MyApplication::WORK_METRIC).to receive(:emit).with(1)
+		MyApplication.new.work
+	end
+end
+```