Add better explanation for how to configure extension classes (open-telemetry#8898)

Co-authored-by: Trask Stalnaker <[email protected]>

Author: tylerbenson

examples/extension/README.md

@@ -34,12 +34,30 @@ For more information, see the `extendedAgent` task in [build.gradle](build.gradl
 
 ## Extensions examples
 
-- Custom `IdGenerator`: [DemoIdGenerator](src/main/java/com/example/javaagent/DemoIdGenerator.java)
-- Custom `TextMapPropagator`: [DemoPropagator](src/main/java/com/example/javaagent/DemoPropagator.java)
-- Custom `Sampler`: [DemoSampler](src/main/java/com/example/javaagent/DemoSampler.java)
-- Custom `SpanProcessor`: [DemoSpanProcessor](src/main/java/com/example/javaagent/DemoSpanProcessor.java)
-- Custom `SpanExporter`: [DemoSpanExporter](src/main/java/com/example/javaagent/DemoSpanExporter.java)
-- Additional instrumentation: [DemoServlet3InstrumentationModule](src/main/java/com/example/javaagent/instrumentation/DemoServlet3InstrumentationModule.java)
+[DemoAutoConfigurationCustomizerProvider]: src/main/java/com/example/javaagent/DemoAutoConfigurationCustomizerProvider.java
+[DemoIdGenerator]: src/main/java/com/example/javaagent/DemoIdGenerator.java
+[DemoPropagator]: src/main/java/com/example/javaagent/DemoPropagator.java
+[DemoSampler]: src/main/java/com/example/javaagent/DemoSampler.java
+[DemoSpanProcessor]: src/main/java/com/example/javaagent/DemoSpanProcessor.java
+[DemoSpanExporter]: src/main/java/com/example/javaagent/DemoSpanExporter.java
+[DemoServlet3InstrumentationModule]: src/main/java/com/example/javaagent/instrumentation/DemoServlet3InstrumentationModule.java
+
+- Custom `AutoConfigurationCustomizer`: [DemoAutoConfigurationCustomizerProvider][DemoAutoConfigurationCustomizerProvider]
+- Custom `IdGenerator`: [DemoIdGenerator][DemoIdGenerator]
+- Custom `TextMapPropagator`: [DemoPropagator][DemoPropagator]
+- Custom `Sampler`: [DemoSampler][DemoSampler]
+- Custom `SpanProcessor`: [DemoSpanProcessor][DemoSpanProcessor]
+- Custom `SpanExporter`: [DemoSpanExporter][DemoSpanExporter]
+- Additional instrumentation: [DemoServlet3InstrumentationModule][DemoServlet3InstrumentationModule]
+
+`ConfigurablePropagatorProvider` and `AutoConfigurationCustomizer` implementations and custom
+instrumentation (`InstrumentationModule`) need the correct SPI (through `@AutoService`) in
+order to be loaded by the agent. Once a `ConfigurablePropagatorProvider` is added, it can be
+referenced by name in the `OTEL_PROPAGATORS` setting. `AutoConfigurationCustomizer` and
+instrumentation will be applied automatically. To apply the other extension classes to the Java
+Agent, include an `AutoConfigurationCustomizer` in your extension.
+See [DemoAutoConfigurationCustomizerProvider][DemoAutoConfigurationCustomizerProvider] for an
+example.
 
 ## Sample use cases