Document observation-related interfaces and classes are SPI

Author: acogoluegnes

src/main/java/com/rabbitmq/client/amqp/ObservationCollector.java

@@ -22,6 +22,8 @@
 /**
  * API to observe common message operations.
  *
+ * <p>This interface is considered an SPI and is susceptible to change at any time.
+ *
  * @see com.rabbitmq.client.amqp.observation.micrometer.MicrometerObservationCollectorBuilder
  */
 public interface ObservationCollector {

src/main/java/com/rabbitmq/client/amqp/observation/micrometer/package-info.java

@@ -3,5 +3,8 @@
  *
  * <p>The entry point is the {@link
  * com.rabbitmq.client.amqp.observation.micrometer.MicrometerObservationCollectorBuilder} class.
+ *
+ * <p>Classes and interfaces in this package are considered SPI and are susceptible to change at any
+ * time.
  */
 package com.rabbitmq.client.amqp.observation.micrometer;