Clarify use of OpenTelemetry's environment variables

See gh-47960

documentation/spring-boot-docs/build.gradle

@@ -150,9 +150,10 @@ dependencies {
 	implementation("io.micrometer:micrometer-tracing")
 	implementation("io.micrometer:micrometer-registry-graphite")
 	implementation("io.micrometer:micrometer-registry-jmx")
-	implementation("io.opentelemetry.instrumentation:opentelemetry-logback-appender-1.0")
-	implementation("io.opentelemetry:opentelemetry-sdk-metrics")
 	implementation("io.opentelemetry:opentelemetry-exporter-otlp")
+	implementation("io.opentelemetry:opentelemetry-sdk-extension-autoconfigure")
+	implementation("io.opentelemetry:opentelemetry-sdk-metrics")
+	implementation("io.opentelemetry.instrumentation:opentelemetry-logback-appender-1.0")
 	implementation("io.projectreactor.netty:reactor-netty-http")
 	implementation("jakarta.annotation:jakarta.annotation-api")
 	implementation("jakarta.jms:jakarta.jms-api")

documentation/spring-boot-docs/src/docs/antora/modules/reference/pages/actuator/observability.adoc

@@ -137,7 +137,17 @@ Micrometer also supports the following environment variables to configure the me
 * https://opentelemetry.io/docs/languages/sdk-configuration/otlp-exporter/#otel_exporter_otlp_headers[`OTEL_EXPORTER_OTLP_HEADERS`]
 * https://opentelemetry.io/docs/languages/sdk-configuration/otlp-exporter/#otel_exporter_otlp_metrics_headers[`OTEL_EXPORTER_OTLP_METRICS_HEADERS`]
 
-Logging and tracing use the OpenTelemetry SDK directly, so all environment variables https://opentelemetry.io/docs/specs/otel/configuration/sdk-environment-variables/[supported by OpenTelemetry] also work.
+Other environment variables as described in https://opentelemetry.io/docs/specs/otel/configuration/sdk-environment-variables/[the OpenTelemetry documentation] are not supported.
+
+If you want all environment variables specified by OpenTelemetry's SDK to be effective, you have to supply your own `OpenTelemetry` bean.
+
+WARNING: Doing this will switch off Spring Boot's OpenTelemetry auto-configuration and may break the built-in observability functionality.
+
+First, add a dependency to `io.opentelemetry:opentelemetry-sdk-extension-autoconfigure` to get https://opentelemetry.io/docs/languages/java/configuration/#zero-code-sdk-autoconfigure[OpenTelemetry's zero-code SDK autoconfigure module], then add this configuration:
+
+include-code::AutoConfiguredOpenTelemetrySdkConfiguration[]
+
+
 
 [[actuator.observability.opentelemetry.logging]]
 === Logging

documentation/spring-boot-docs/src/main/java/org/springframework/boot/docs/actuator/observability/opentelemetry/environmentvariables/AutoConfiguredOpenTelemetrySdkConfiguration.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2012-present the original author or authors.
+ *
+ * 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.
+ */
+
+package org.springframework.boot.docs.actuator.observability.opentelemetry.environmentvariables;
+
+import io.opentelemetry.api.OpenTelemetry;
+import io.opentelemetry.sdk.autoconfigure.AutoConfiguredOpenTelemetrySdk;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+@Configuration(proxyBeanMethods = false)
+class AutoConfiguredOpenTelemetrySdkConfiguration {
+
+	@Bean
+	OpenTelemetry autoConfiguredOpenTelemetrySdk() {
+		return AutoConfiguredOpenTelemetrySdk.initialize().getOpenTelemetrySdk();
+	}
+
+}