Ensure that the running agent's config is used when restarted (#763)

* Ensure that the running agent's config is used when restarted

* Add changelog entry

CHANGELOG.asciidoc

@@ -50,6 +50,7 @@ endif::[]
 
 - Ensure that the log level is updated for the config's logger when value is changed {pull}755[#755]
 - Set config `false` values to `false`, not `nil` {pull}761[#761]
+- Ensure that the previously running agent's config is used in `ElasticAPM.restart` {pull}763[#763]
 
 [float]
 ===== Added

lib/elastic_apm.rb

@@ -51,7 +51,7 @@ def stop
     # one, if it is provided.
     # Starts the agent if it is not running.
     # Stops and starts the agent if it is running.
-    def restart(config = {})
+    def restart(config = nil)
       config ||= agent&.config
       stop if running?
       start(config)

spec/elastic_apm_spec.rb

@@ -36,12 +36,21 @@
       end
     end
     context 'when a new config is passed' do
+      before { ElasticAPM.start }
       it 'restarts the agent with the new config' do
         ElasticAPM.restart(api_buffer_size: 10)
         expect(ElasticAPM::Agent).to be_running
         expect(ElasticAPM.agent.config.api_buffer_size).to be(10)
       end
     end
+    context 'when no new config is passed' do
+      before { ElasticAPM.start(api_buffer_size: 10) }
+      it 'restarts the agent with the same config' do
+        ElasticAPM.restart
+        expect(ElasticAPM::Agent).to be_running
+        expect(ElasticAPM.agent.config.api_buffer_size).to be(10)
+      end
+    end
   end
 
   context 'when running', :mock_intake do