Add dsn_url method to client key

Generate URL for sentry clients based on public key and project id from
client key
Signed-off-by: Dmytro Zaporozhets <dzaporozhets@gitlab.com>

app/models/error_tracking/client_key.rb

@@ -14,6 +14,10 @@ class ErrorTracking::ClientKey < ApplicationRecord
     find_by(public_key: key)
   end
 
+  def sentry_dsn
+    @sentry_dsn ||= ErrorTracking::Collector::Dsn.build_url(public_key, project_id)
+  end
+
   private
 
   def generate_key

lib/error_tracking/collector/dsn.rb

+# frozen_string_literal: true
+
+module ErrorTracking
+  module Collector
+    class Dsn
+      # Build a sentry compatible DSN URL for GitLab collector.
+      #
+      # The expected URL looks like that:
+      #   https://PUBLIC_KEY@gitlab.example.com/api/v4/error_tracking/collector/PROJECT_ID
+      #
+      def self.build_url(public_key, project_id)
+        gitlab = Settings.gitlab
+
+        custom_port = Settings.gitlab_on_standard_port? ? nil : ":#{gitlab.port}"
+
+        base_url = [
+          gitlab.protocol,
+          "://",
+          public_key,
+          '@',
+          gitlab.host,
+          custom_port,
+          gitlab.relative_url_root
+        ].join('')
+
+        "#{base_url}/api/v4/error_tracking/collector/#{project_id}"
+      end
+    end
+  end
+end

spec/lib/error_tracking/collector/dsn_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe ErrorTracking::Collector::Dsn do
+  describe '.build__url' do
+    let(:gitlab) do
+      double(
+        protocol: 'https',
+        https: true,
+        host: 'gitlab.example.com',
+        port: '4567',
+        relative_url_root: nil
+      )
+    end
+
+    subject { described_class.build_url('abcdef1234567890', 778) }
+
+    it 'returns a valid URL' do
+      allow(Settings).to receive(:gitlab).and_return(gitlab)
+      allow(Settings).to receive(:gitlab_on_standard_port?).and_return(false)
+
+      is_expected.to eq('https://abcdef1234567890@gitlab.example.com:4567/api/v4/error_tracking/collector/778')
+    end
+  end
+end