Provide a performance bar link to the Jaeger UI

Jaeger is a distributed tracing tool. This change adds a "Tracing" link
to the performance bar to directly link to a current request in Jaeger.

This is useful for two reasons: 1 - it provides affordance to developers
that the distributed tracing tool is available, so that it can quickly
be discovered. 2 - it allows developers to quickly find a specific trace
without having to manually navigate to a second user-interface.

app/assets/javascripts/performance_bar/components/performance_bar_app.vue

@@ -134,6 +134,13 @@ export default {
           >ms / <span title="Invoke Count">{{ currentRequest.details.gc.invokes }}</span> gc
         </span>
       </div>
+      <div
+        v-if="currentRequest.details && currentRequest.details.tracing"
+        id="peek-view-trace"
+        class="view"
+      >
+        <a :href="currentRequest.details.tracing.tracing_url"> trace </a>
+      </div>
       <request-selector
         v-if="currentRequest"
         :current-request="currentRequest"

changelogs/unreleased/an-peek-jaeger.yml

+---
+title: Provide a performance bar link to the Jaeger UI
+merge_request: 24902
+author:
+type: other

config/initializers/peek.rb

@@ -19,6 +19,7 @@ Peek.into Peek::Views::Gitaly
 Peek.into Peek::Views::Rblineprof
 Peek.into Peek::Views::Redis
 Peek.into Peek::Views::GC
+Peek.into Peek::Views::Tracing if Gitlab::Tracing.tracing_url_enabled?
 
 # rubocop:disable Naming/ClassAndModuleCamelCase
 class PEEK_DB_CLIENT

lib/gitlab/tracing.rb

@@ -13,5 +13,24 @@ module Gitlab
     def self.connection_string
       ENV['GITLAB_TRACING']
     end
+
+    def self.tracing_url_template
+      ENV['GITLAB_TRACING_URL']
+    end
+
+    def self.tracing_url_enabled?
+      enabled? && tracing_url_template.present?
+    end
+
+    # This will provide a link into the distributed tracing for the current trace,
+    # if it has been captured.
+    def self.tracing_url
+      return unless tracing_url_enabled?
+
+      tracing_url_template % {
+        correlation_id: Gitlab::CorrelationId.current_id.to_s,
+        service: Gitlab.process_name
+      }
+    end
   end
 end

lib/peek/views/tracing.rb

+# frozen_string_literal: true
+
+module Peek
+  module Views
+    class Tracing < View
+      def results
+        {
+          tracing_url: Gitlab::Tracing.tracing_url
+        }
+      end
+    end
+  end
+end

spec/lib/gitlab/tracing_spec.rb

+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+require 'rspec-parameterized'
+
+describe Gitlab::Tracing do
+  using RSpec::Parameterized::TableSyntax
+
+  describe '.enabled?' do
+    where(:connection_string, :enabled_state) do
+      nil                     | false
+      ""                      | false
+      "opentracing://jaeger"  | true
+    end
+
+    with_them do
+      it 'should return the correct state for .enabled?' do
+        expect(described_class).to receive(:connection_string).and_return(connection_string)
+
+        expect(described_class.enabled?).to eq(enabled_state)
+      end
+    end
+  end
+
+  describe '.tracing_url_enabled?' do
+    where(:enabled?, :tracing_url_template, :tracing_url_enabled_state) do
+      false | nil                | false
+      false | ""                 | false
+      false | "http://localhost" | false
+      true  | nil                | false
+      true  | ""                 | false
+      true  | "http://localhost" | true
+    end
+
+    with_them do
+      it 'should return the correct state for .tracing_url_enabled?' do
+        expect(described_class).to receive(:enabled?).and_return(enabled?)
+        allow(described_class).to receive(:tracing_url_template).and_return(tracing_url_template)
+
+        expect(described_class.tracing_url_enabled?).to eq(tracing_url_enabled_state)
+      end
+    end
+  end
+
+  describe '.tracing_url' do
+    where(:tracing_url_enabled?, :tracing_url_template, :correlation_id, :process_name, :tracing_url) do
+      false | "https://localhost"                                  | "123" | "web" | nil
+      true  | "https://localhost"                                  | "123" | "web" | "https://localhost"
+      true  | "https://localhost?service=%{service}"               | "123" | "web" | "https://localhost?service=web"
+      true  | "https://localhost?c=%{correlation_id}"              | "123" | "web" | "https://localhost?c=123"
+      true  | "https://localhost?c=%{correlation_id}&s=%{service}" | "123" | "web" | "https://localhost?c=123&s=web"
+      true  | "https://localhost?c=%{correlation_id}"              | nil   | "web" | "https://localhost?c="
+    end
+
+    with_them do
+      it 'should return the correct state for .tracing_url' do
+        expect(described_class).to receive(:tracing_url_enabled?).and_return(tracing_url_enabled?)
+        allow(described_class).to receive(:tracing_url_template).and_return(tracing_url_template)
+        allow(Gitlab::CorrelationId).to receive(:current_id).and_return(correlation_id)
+        allow(Gitlab).to receive(:process_name).and_return(process_name)
+
+        expect(described_class.tracing_url).to eq(tracing_url)
+      end
+    end
+  end
+end