Change OTel quickstart to use log4j2 instead of logback (#330)

* Change OTel quickstart to use log4j2 instead of logback

Log4j2's `JsonTemplateLayout` supports most of the Cloud Logging special JSON keys directly through the [`GcpLayout` event template](https://logging.apache.org/log4j/2.x/manual/json-template-layout.html#event-templates). I still needed to map the OTel MDC keys for trace correlation, but am planning to contribute [this upstream](https://github.com/apache/logging-log4j2/blob/rel/2.17.2/log4j-layout-template-json/src/main/resources/GcpLayout.json).

One outstanding issue is converting the `trace_flags` hex value to a boolean, but it doesn't break Cloud Console if `logging.googleapis.com/trace_sampled` key is unset/false. See apache/logging-log4j2#2482.

* undo one collector config change, add issue ID to TODO

examples/instrumentation-quickstart/build.gradle

@@ -42,9 +42,15 @@ spotless {
 dependencies {
 	implementation 'org.springframework.boot:spring-boot-starter-webflux'
 
-	// Cannot be updated until logback is updated to 1.3+, probably in the next Spring Boot
-	// major version
-	implementation 'net.logstash.logback:logstash-logback-encoder:7.3'
+	// Use log4j2 for logging
+	// https://docs.spring.io/spring-boot/docs/2.7.18/reference/html/howto.html#howto.logging.log4j
+	implementation "org.springframework.boot:spring-boot-starter-log4j2"
+	modules {
+		module("org.springframework.boot:spring-boot-starter-logging") {
+			replacedBy("org.springframework.boot:spring-boot-starter-log4j2", "Use Log4j2 instead of Logback")
+		}
+	}
+	implementation "org.apache.logging.log4j:log4j-layout-template-json"
 
 	testImplementation 'org.springframework.boot:spring-boot-starter-test'
 	testImplementation 'org.testcontainers:testcontainers:1.19.4'

examples/instrumentation-quickstart/otel-collector-config.yaml

@@ -35,6 +35,18 @@ receivers:
           layout: '%Y-%m-%dT%H:%M:%S.%fZ'
         severity:
           parse_from: body.severity
+          preset: none
+          # parse minimal set of severity strings that Cloud Logging explicitly supports
+          # https://cloud.google.com/logging/docs/reference/v2/rest/v2/LogEntry#LogSeverity
+          mapping:
+            debug: debug
+            info: info
+            info3: notice
+            warn: warning
+            error: error
+            fatal: critical
+            fatal3: alert
+            fatal4: emergency
 
       # set trace_flags to SAMPLED if GCP attribute is set to true
       - type: add

examples/instrumentation-quickstart/src/main/resources/log4j2.xml

@@ -0,0 +1,49 @@
+<!--
+ Copyright 2024 Google LLC
+
+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use this file except in compliance with the License.
+ You may obtain a copy of the License at
+
+  https://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+ -->
+
+<Configuration status="WARN">
+  <Appenders>
+    <Console name="Console" target="SYSTEM_OUT">
+      <!-- TODO(#331): output logging.googleapis.com/trace_sampled as a boolean -->
+      <!-- [START opentelemetry_instrumentation_setup_logging] -->
+      <!-- Format JSON logs for the Cloud Logging agent
+      https://cloud.google.com/logging/docs/structured-logging#special-payload-fields -->
+
+      <!-- Log4j2's JsonTemplateLayout includes a template for Cloud Logging's special JSON fields
+      https://logging.apache.org/log4j/2.x/manual/json-template-layout.html#event-templates -->
+      <JsonTemplateLayout eventTemplateUri="classpath:GcpLayout.json">
+        <!-- Extend the included GcpLayout to include the trace and span IDs from Mapped
+        Diagnostic Context (MDC) so that Cloud Logging can correlate Logs and Spans -->
+        <EventTemplateAdditionalField
+          key="logging.googleapis.com/trace"
+          format="JSON"
+          value='{"$resolver": "mdc", "key": "trace_id"}'
+        />
+        <EventTemplateAdditionalField
+          key="logging.googleapis.com/spanId"
+          format="JSON"
+          value='{"$resolver": "mdc", "key": "span_id"}'
+        />
+      </JsonTemplateLayout>
+      <!-- [END opentelemetry_instrumentation_setup_logging] -->
+    </Console>
+  </Appenders>
+  <Loggers>
+    <Root level="info">
+      <AppenderRef ref="Console" />
+    </Root>
+  </Loggers>
+</Configuration>