fix env var override for spring starter declarative config

instrumentation/spring/spring-boot-autoconfigure/build.gradle.kts

@@ -180,6 +180,7 @@ testing {
       dependencies {
         implementation(project())
         implementation("io.opentelemetry:opentelemetry-sdk")
+        implementation("io.opentelemetry:opentelemetry-exporter-otlp")
         implementation("org.springframework.boot:spring-boot-starter-test:$springBootVersion") {
           exclude("org.junit.vintage", "junit-vintage-engine")
         }

instrumentation/spring/spring-boot-autoconfigure/src/main/java/io/opentelemetry/instrumentation/spring/autoconfigure/EmbeddedConfigFile.java

@@ -11,6 +11,7 @@
 import io.opentelemetry.sdk.extension.incubator.fileconfig.internal.model.OpenTelemetryConfigurationModel;
 import java.util.ArrayList;
 import java.util.HashMap;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Objects;
 import java.util.regex.Matcher;
@@ -62,6 +63,24 @@ private static Map<String, String> extractSpringProperties(ConfigurableEnvironme
             if (Objects.equals(property, "")) {
               property = null; // spring returns empty string for yaml null
             }
+            if (propertyName.contains("[")) {
+              // fix override via env var or system property
+              // see
+              // https://docs.spring.io/spring-boot/reference/features/external-config.html#features.external-config.typesafe-configuration-properties.relaxed-binding
+              // For example, the configuration property my.service[0].other would use an
+              // environment variable named MY_SERVICE_0_OTHER.
+              String envVarName =
+                  propertyName
+                      .replace("[", "_")
+                      .replace("]", "")
+                      .replace(".", "_")
+                      .toUpperCase(Locale.ROOT);
+              String envVarValue = environment.getProperty(envVarName);
+              if (envVarValue != null) {
+                property = envVarValue;
+              }
+            }
+
             props.put(propertyName.substring("otel.".length()), property);
           }
         }

instrumentation/spring/spring-boot-autoconfigure/src/testDeclarativeConfig/java/io/opentelemetry/instrumentation/spring/autoconfigure/DeclarativeConfigTest.java

@@ -131,4 +131,23 @@ void shouldNotLoadInstrumentationWhenExplicitlyDisabled() {
             "otel.instrumentation/development.java.spring_web.enabled=false")
         .run(context -> assertThat(context).doesNotHaveBean("otelRestTemplateBeanPostProcessor"));
   }
+
+  @Test
+  void envVarOverride() {
+    this.contextRunner
+        // this is typically set via env var
+        // OTEL_TRACER_PROVIDER_PROCESSORS_0_BATCH_EXPORTER_OTLP_HTTP_ENDPOINT
+        .withSystemProperties(
+            "OTEL_TRACER_PROVIDER_PROCESSORS_0_BATCH_EXPORTER_OTLP_HTTP_ENDPOINT=http://custom:4318/v1/traces")
+        .run(
+            context ->
+                assertThat(context)
+                    .getBean(OpenTelemetry.class)
+                    .isNotNull()
+                    .satisfies(
+                        c ->
+                            assertThat(c.toString())
+                                .contains(
+                                    "OtlpHttpSpanExporter{endpoint=http://custom:4318/v1/traces")));
+  }
 }

instrumentation/spring/spring-boot-autoconfigure/src/testDeclarativeConfig/resources/application.yaml

@@ -2,6 +2,13 @@ otel:
   # "file_format" serves as opt-in to the new file format
   file_format: "1.0-rc.1"
 
+  tracer_provider:
+    processors:
+      - batch:
+          exporter:
+            otlp_http:
+              endpoint: http://localhost:4318/v1/traces # to test that we can use env vars in declarative config
+
   # very lightweight test to make sure the declarative config is loaded
   # the full config is tested in smoke-tests-otel-starter/spring-boot-2/src/testDeclarativeConfig
   instrumentation/development: