(POOLER-177) Filter hostname from API Paths

Use the example provided in the Ruby Client to provide a customised
collector appropriate to log all calls to the API. The customised
filtering is used to replace individual node names and templates
for the /vm and request ID's for the /ondemand endpoints.

This module was failing our rubocop checks so have updated it since
it now forms part of vmpooler.

Separate trapping for litmus jobs is also included so that they don't
interfere with stats from the jenkins pipelines.

spec/unit/collector_middleware_spec.rb

@@ -0,0 +1,135 @@
+
+require 'rack/test'
+require 'vmpooler/metrics/promstats/collector_middleware'
+
+
+describe Vmpooler::Metrics::Promstats::CollectorMiddleware do
+  include Rack::Test::Methods
+
+  # Reset the data store
+  before do
+    Prometheus::Client.config.data_store = Prometheus::Client::DataStores::Synchronized.new
+  end
+
+  let(:registry) do
+    Prometheus::Client::Registry.new
+  end
+
+  let(:original_app) do
+    ->(_) { [200, { 'Content-Type' => 'text/html' }, ['OK']] }
+  end
+
+  let!(:app) do
+    described_class.new(original_app, registry: registry)
+  end
+
+  let(:dummy_error) { RuntimeError.new("Dummy error from tests") }
+
+  it 'returns the app response' do
+    get '/foo'
+
+    expect(last_response).to be_ok
+    expect(last_response.body).to eql('OK')
+  end
+
+  it 'handles errors in the registry gracefully' do
+    counter = registry.get(:http_server_requests_total)
+    expect(counter).to receive(:increment).and_raise(dummy_error)
+
+    get '/foo'
+
+    expect(last_response).to be_ok
+  end
+
+  it 'traces request information' do
+    expect(Benchmark).to receive(:realtime).and_yield.and_return(0.2)
+
+    get '/foo'
+
+    metric = :http_server_requests_total
+    labels = { method: 'get', path: '/foo', code: '200' }
+    expect(registry.get(metric).get(labels: labels)).to eql(1.0)
+
+    metric = :http_server_request_duration_seconds
+    labels = { method: 'get', path: '/foo' }
+    expect(registry.get(metric).get(labels: labels)).to include("0.1" => 0, "0.25" => 1)
+  end
+
+  it 'normalizes paths cotaining /vm by default' do
+    expect(Benchmark).to receive(:realtime).and_yield.and_return(0.3)
+
+    get '/foo/vm/bar-mumble-flame'
+
+    metric = :http_server_requests_total
+    labels = { method: 'get', path: '/foo/vm', code: '200' }
+    expect(registry.get(metric).get(labels: labels)).to eql(1.0)
+
+    metric = :http_server_request_duration_seconds
+    labels = { method: 'get', path: '/foo/vm' }
+    expect(registry.get(metric).get(labels: labels)).to include("0.1" => 0, "0.5" => 1)
+  end
+
+  it 'normalizes paths containing /ondemandvm by ' do
+    expect(Benchmark).to receive(:realtime).and_yield.and_return(0.3)
+
+    get '/foo/ondemand/bar/fatman'
+
+    metric = :http_server_requests_total
+    labels = { method: 'get', path: '/foo/ondemand', code: '200' }
+    expect(registry.get(metric).get(labels: labels)).to eql(1.0)
+
+    metric = :http_server_request_duration_seconds
+    labels = { method: 'get', path: '/foo/ondemand' }
+    expect(registry.get(metric).get(labels: labels)).to include("0.1" => 0, "0.5" => 1)
+  end
+
+  context 'when the app raises an exception' do
+    let(:original_app) do
+      lambda do |env|
+        raise dummy_error if env['PATH_INFO'] == '/broken'
+
+        [200, { 'Content-Type' => 'text/html' }, ['OK']]
+      end
+    end
+
+    before do
+      get '/foo'
+    end
+
+    it 'traces exceptions' do
+      expect { get '/broken' }.to raise_error RuntimeError
+
+      metric = :http_server_exceptions_total
+      labels = { exception: 'RuntimeError' }
+      expect(registry.get(metric).get(labels: labels)).to eql(1.0)
+    end
+  end
+
+  context 'when provided a custom metrics_prefix' do
+    let!(:app) do
+      described_class.new(
+        original_app,
+        registry: registry,
+        metrics_prefix: 'lolrus',
+      )
+    end
+
+    it 'provides alternate metric names' do
+      expect(
+        registry.get(:lolrus_requests_total),
+      ).to be_a(Prometheus::Client::Counter)
+      expect(
+        registry.get(:lolrus_request_duration_seconds),
+      ).to be_a(Prometheus::Client::Histogram)
+      expect(
+        registry.get(:lolrus_exceptions_total),
+      ).to be_a(Prometheus::Client::Counter)
+    end
+
+    it "doesn't register the default metrics" do
+      expect(registry.get(:http_server_requests_total)).to be(nil)
+      expect(registry.get(:http_server_request_duration_seconds)).to be(nil)
+      expect(registry.get(:http_server_exceptions_total)).to be(nil)
+    end
+  end
+end