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

@@ -0,0 +1,20 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.javaagent.tooling.config;
+
+import io.opentelemetry.instrumentation.api.config.Config;
+import io.opentelemetry.javaagent.extension.Ordered;
+
+/**
+ * Internal SPI that allows to 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 {
+
+  Config customize(Config config);
+}

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

@@ -27,7 +27,11 @@ public final class ConfigInitializer {
   static final String CONFIGURATION_FILE_ENV_VAR = "OTEL_JAVAAGENT_CONFIGURATION_FILE";
 
   public static void initialize() {
-    Config.internalInitializeConfig(create(loadSpiConfiguration(), loadConfigurationFile()));
+    Config config = create(loadSpiConfiguration(), loadConfigurationFile());
+    for (ConfigCustomizer customizer : loadOrdered(ConfigCustomizer.class)) {
+      config = customizer.customize(config);
+    }
+    Config.internalInitializeConfig(config);
   }
 
   // visible for testing