Moved API to internal directory

Author: steverao

docs/apidiffs/current_vs_latest/opentelemetry-instrumentation-api.txt

@@ -1,5 +1,5 @@
 Comparing source compatibility of opentelemetry-instrumentation-api-2.17.0-SNAPSHOT.jar against opentelemetry-instrumentation-api-2.16.0.jar
-+++  NEW INTERFACE: PUBLIC(+) ABSTRACT(+) io.opentelemetry.instrumentation.api.instrumenter.InstrumentationCustomizer  (not serializable)
++++  NEW INTERFACE: PUBLIC(+) ABSTRACT(+) io.opentelemetry.instrumentation.api.internal.InstrumentationCustomizer  (not serializable)
 	+++  CLASS FILE FORMAT VERSION: 52.0 <- n.a.
 	+++  NEW SUPERCLASS: java.lang.Object
 	+++  NEW METHOD: PUBLIC(+) io.opentelemetry.instrumentation.api.instrumenter.AttributesExtractor<REQUEST,RESPONSE> getAttributesExtractor()

instrumentation-api/src/main/java/io/opentelemetry/instrumentation/api/instrumenter/InstrumenterBuilder.java

@@ -21,6 +21,7 @@
 import io.opentelemetry.context.propagation.TextMapSetter;
 import io.opentelemetry.instrumentation.api.internal.ConfigPropertiesUtil;
 import io.opentelemetry.instrumentation.api.internal.EmbeddedInstrumentationProperties;
+import io.opentelemetry.instrumentation.api.internal.InstrumentationCustomizer;
 import io.opentelemetry.instrumentation.api.internal.InstrumenterBuilderAccess;
 import io.opentelemetry.instrumentation.api.internal.InstrumenterUtil;
 import io.opentelemetry.instrumentation.api.internal.SchemaUrlProvider;

instrumentation-api/src/main/java/io/opentelemetry/instrumentation/api/instrumenter/InstrumentationCustomizer.java renamed to instrumentation-api/src/main/java/io/opentelemetry/instrumentation/api/internal/InstrumentationCustomizer.java

@@ -3,16 +3,20 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-package io.opentelemetry.instrumentation.api.instrumenter;
+package io.opentelemetry.instrumentation.api.internal;
 
+import io.opentelemetry.instrumentation.api.instrumenter.AttributesExtractor;
+import io.opentelemetry.instrumentation.api.instrumenter.ContextCustomizer;
+import io.opentelemetry.instrumentation.api.instrumenter.OperationMetrics;
 import java.util.function.Predicate;
 
 /**
  * A service provider interface (SPI) for providing customizations for instrumentation, including
  * operation metrics, attributes extraction, and context customization.
  *
  * <p>This allows external modules or plugins to contribute custom logic for specific instrumented
- * libraries, without modifying core instrumentation code.
+ * libraries, without modifying core instrumentation code. This class is internal and is hence not
+ * for public use. Its APIs are unstable and can change at any time.
  */
 public interface InstrumentationCustomizer {