create example for baggage

examples/baggage/README.md

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+# Baggage examples
+
+This is an elementary application providing an end-to-end example usage of [OpenTelemetry baggage](https://opentelemetry.io/docs/concepts/signals/baggage/).
+
+It is split in two services: a gateway and a backend
+- an HTTP backend service running on http://localhost:9000/backend/
+- an HTTP gateway service running on http://localhost:8000/gateway/
+
+The gateway is in charge of authentication and delegates to the backend by providing a technical ID for the backend to use for authentication.
+
+## Build
+
+```bash
+./gradlew clean assemble
+```
+
+## Configure
+
+The javaagent is already packaged and included in the application by using [runtime attach](https://www.elastic.co/docs/reference/opentelemetry/edot-sdks/java/setup/runtime-attach) feature.
+
+OTel configuration must be provided through environment variables or JVM system properties.
+
+## Run
+
+Backend:
+```shell
+export OTEL_SERVICE_NAME='backend'
+java \
+-Dotel.java.experimental.span-attributes.copy-from-baggage.include=example.customer.id,example.customer.name \
+-Dotel.java.experimental.log-attributes.copy-from-baggage.include=example.customer.id,example.customer.name \
+-jar ./build/libs/baggage-example-all.jar backend
+```
+
+Gateway:
+```shell
+export OTEL_SERVICE_NAME='gateway'
+java \
+-Dotel.java.experimental.span-attributes.copy-from-baggage.include=example.customer.id,example.customer.name \
+-Dotel.java.experimental.log-attributes.copy-from-baggage.include=example.customer.id,example.customer.name \
+-jar ./build/libs/baggage-example-all.jar gateway
+```
+
+Once both services are started, you can execute queries on the gateway with queries like the following:
+```shell
+curl -H 'Authorization: secret=12345' http://localhost:8000/gateway/
+```
+
+This will create the following log messages on `gateway` and `backend` respectively:
+```
+gateway request for customer ID = 12345
+backend request for customer ID = 12345
+```
+
+All the spans and logs captured within the scope of the baggage will have the following attributes:
+- `example.customer.name` with value `my-awesome-customer-12345`
+- `example.customer.id` with value `12345`
+
+As a result, it is now possible to use those custom attributes to filter data and create
+dedicated dashboards. The only code modification required is in the gateway code.
+

examples/baggage/build.gradle.kts

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+plugins {
+  application
+  java
+
+  id("com.gradleup.shadow") version "8.3.5"
+}
+
+group = "baggage.example"
+version = "1.0-SNAPSHOT"
+
+repositories {
+  mavenCentral()
+}
+
+dependencies {
+  implementation("co.elastic.otel:elastic-otel-runtime-attach:1.4.1")
+  implementation(platform("org.slf4j:slf4j-bom:2.0.16"))
+
+  // using a "real" logger backend as slf4j-simple just uses stdout/stderr
+  // and is not instrumented for log capture by otel instrumentation
+  implementation("org.apache.logging.log4j:log4j-slf4j2-impl:2.25.0")
+
+  // otel API to access Baggage API
+  implementation("io.opentelemetry:opentelemetry-api:1.51.0")
+
+}
+
+application {
+  mainClass = "baggage.example.Main"
+}
+
+tasks {
+  compileJava {
+    options.release.set(21)
+  }
+
+  shadowJar {
+    archiveFileName.set("baggage-example-all.jar")
+  }
+
+  assemble {
+    dependsOn("shadowJar")
+  }
+}

examples/baggage/gradle/wrapper/gradle-wrapper.properties

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+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionSha256Sum=20f1b1176237254a6fc204d8434196fa11a4cfb387567519c61556e8710aed78
+distributionUrl=https\://services.gradle.org/distributions/gradle-8.13-bin.zip
+networkTimeout=10000
+validateDistributionUrl=true
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists