Add an SPI for customizing Config just before it's set

instrumentation-api/src/main/java/io/opentelemetry/instrumentation/api/config/Config.java

@@ -192,4 +192,12 @@ private <T> T getTypedProperty(String name, ConfigValueParser<T> parser) {
   private String getRawProperty(String name, String defaultValue) {
     return getAllProperties().getOrDefault(NamingConvention.DOT.normalize(name), defaultValue);
   }
+
+  /**
+   * Returns a new {@link ConfigBuilder} instance populated with the properties of this {@link
+   * Config}.
+   */
+  public ConfigBuilder toBuilder() {
+    return new ConfigBuilder(getAllProperties());
+  }
 }

instrumentation-api/src/main/java/io/opentelemetry/instrumentation/api/config/ConfigBuilder.java

@@ -14,10 +14,15 @@
 /** A builder of a {@link Config}. */
 public final class ConfigBuilder {
 
-  private final Map<String, String> allProperties = new HashMap<>();
+  private final Map<String, String> allProperties;
 
-  /** Constructs a new {@link ConfigBuilder}. */
-  ConfigBuilder() {}
+  ConfigBuilder() {
+    allProperties = new HashMap<>();
+  }
+
+  ConfigBuilder(Map<String, String> propertiesToCopy) {
+    allProperties = new HashMap<>(propertiesToCopy);
+  }
 
   /** Adds a single property to the config. */
   public ConfigBuilder addProperty(String name, @Nullable String value) {

javaagent-extension-api/src/main/java/io/opentelemetry/javaagent/extension/config/ConfigCustomizer.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.javaagent.extension.config;
+
+import io.opentelemetry.instrumentation.api.config.Config;
+import io.opentelemetry.javaagent.extension.Ordered;
+import java.util.Collections;
+import java.util.Map;
+
+/**
+ * A service provider that allows to override default OTel javaagent configuration, and customize
+ * the config just before it is set as the global.
+ *
+ * <p>This is a service provider interface that requires implementations to be registered in a
+ * provider-configuration file stored in the {@code META-INF/services} resource directory.
+ */
+public interface ConfigCustomizer extends Ordered {
+
+  /**
+   * Returns properties with their default values. Properties returned by implementations of this
+   * interface will be used after the following methods fail to find a non-empty property value:
+   * system properties, environment variables, properties configuration file.
+   *
+   * <p>Key of the map is the propertyName (same as system property name, e.g. {@code
+   * otel.traces.exporter}), value is the property value.
+   */
+  default Map<String, String> defaultProperties() {
+    return Collections.emptyMap();
+  }
+
+  /** Allows to change the javaagent configuration just before it is first used. */
+  default Config customize(Config config) {
+    return config;
+  }
+}

javaagent-extension-api/src/main/java/io/opentelemetry/javaagent/extension/config/ConfigPropertySource.java

@@ -16,12 +16,19 @@
  *
  * <p>This is a service provider interface that requires implementations to be registered in a
  * provider-configuration file stored in the {@code META-INF/services} resource directory.
+ *
+ * @deprecated Use {@link ConfigCustomizer} instead.
  */
+@Deprecated
 public interface ConfigPropertySource extends Ordered {
+
   /**
    * Returns all properties whose default values are overridden by this property source. Key of the
    * map is the propertyName (same as system property name, e.g. {@code otel.traces.exporter}),
    * value is the property value.
+   *
+   * @deprecated Use {@link ConfigCustomizer#defaultProperties()} instead.
    */
+  @Deprecated
   Map<String, String> getProperties();
 }

javaagent-tooling/src/main/java/io/opentelemetry/javaagent/tooling/config/ConfigInitializer.java

@@ -9,13 +9,14 @@
 import static java.util.logging.Level.SEVERE;
 
 import io.opentelemetry.instrumentation.api.config.Config;
-import io.opentelemetry.javaagent.extension.config.ConfigPropertySource;
+import io.opentelemetry.javaagent.extension.config.ConfigCustomizer;
 import java.io.File;
 import java.io.FileInputStream;
 import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.io.InputStreamReader;
 import java.nio.charset.StandardCharsets;
+import java.util.List;
 import java.util.Properties;
 import java.util.logging.Logger;
 
@@ -27,7 +28,12 @@ public final class ConfigInitializer {
   static final String CONFIGURATION_FILE_ENV_VAR = "OTEL_JAVAAGENT_CONFIGURATION_FILE";
 
   public static void initialize() {
-    Config.internalInitializeConfig(create(loadSpiConfiguration(), loadConfigurationFile()));
+    List<ConfigCustomizer> customizers = loadOrdered(ConfigCustomizer.class);
+    Config config = create(loadSpiConfiguration(customizers), loadConfigurationFile());
+    for (ConfigCustomizer customizer : customizers) {
+      config = customizer.customize(config);
+    }
+    Config.internalInitializeConfig(config);
   }
 
   // visible for testing
@@ -41,11 +47,16 @@ static Config create(Properties spiConfiguration, Properties configurationFile)
   }
 
   /** Retrieves all default configuration overloads using SPI and initializes Config. */
-  private static Properties loadSpiConfiguration() {
+  @SuppressWarnings("deprecation") // loads the old config SPI
+  private static Properties loadSpiConfiguration(List<ConfigCustomizer> customizers) {
     Properties propertiesFromSpi = new Properties();
-    for (ConfigPropertySource propertySource : loadOrdered(ConfigPropertySource.class)) {
+    for (io.opentelemetry.javaagent.extension.config.ConfigPropertySource propertySource :
+        loadOrdered(io.opentelemetry.javaagent.extension.config.ConfigPropertySource.class)) {
       propertiesFromSpi.putAll(propertySource.getProperties());
     }
+    for (ConfigCustomizer customizer : customizers) {
+      propertiesFromSpi.putAll(customizer.defaultProperties());
+    }
     return propertiesFromSpi;
   }
 

testing/agent-exporter/src/main/java/io/opentelemetry/javaagent/testing/exporter/AgentTestingExporterPropertySource.java

@@ -6,14 +6,15 @@
 package io.opentelemetry.javaagent.testing.exporter;
 
 import com.google.auto.service.AutoService;
-import io.opentelemetry.javaagent.extension.config.ConfigPropertySource;
+import io.opentelemetry.javaagent.extension.config.ConfigCustomizer;
 import java.util.HashMap;
 import java.util.Map;
 
-@AutoService(ConfigPropertySource.class)
-public class AgentTestingExporterPropertySource implements ConfigPropertySource {
+@AutoService(ConfigCustomizer.class)
+public class AgentTestingExporterPropertySource implements ConfigCustomizer {
+
   @Override
-  public Map<String, String> getProperties() {
+  public Map<String, String> defaultProperties() {
     Map<String, String> properties = new HashMap<>();
     properties.put("otel.logs.exporter", "none");
     properties.put("otel.metrics.exporter", "none");

testing/agent-exporter/src/main/java/io/opentelemetry/javaagent/testing/http/CapturedHttpHeadersTestConfigSource.java

@@ -6,15 +6,15 @@
 package io.opentelemetry.javaagent.testing.http;
 
 import com.google.auto.service.AutoService;
-import io.opentelemetry.javaagent.extension.config.ConfigPropertySource;
+import io.opentelemetry.javaagent.extension.config.ConfigCustomizer;
 import java.util.HashMap;
 import java.util.Map;
 
-@AutoService(ConfigPropertySource.class)
-public class CapturedHttpHeadersTestConfigSource implements ConfigPropertySource {
+@AutoService(ConfigCustomizer.class)
+public class CapturedHttpHeadersTestConfigSource implements ConfigCustomizer {
 
   @Override
-  public Map<String, String> getProperties() {
+  public Map<String, String> defaultProperties() {
     Map<String, String> testConfig = new HashMap<>();
     testConfig.put("otel.instrumentation.http.capture-headers.client.request", "X-Test-Request");
     testConfig.put("otel.instrumentation.http.capture-headers.client.response", "X-Test-Response");