[opamp-client] Add the ability to receive custom messages

[breedx-splk]

The OpAMP protocol declares the ability to support custom messages.

This PR adds this custom capability. For users, it will look something like this:

OpampClient client =
      OpampClient.builder()
          .addCustomCapability("com.example.pause")
          .build(
              new OpampClient.Callbacks() {
                @Override
                public void onMessage(OpampClient client, MessageData messageData) {
                  CustomMessage customMessage = messageData.getCustomMessage();
                  if (customMessage != null) {
                    if ("com.example.pause".equals(customMessage.capability)
                        && "pause".equals(customMessage.type)) {
                      byte[] payload = customMessage.data.toByteArray();
                      // do the dew...
                    }
                  }
                }
                // ((other message handling omitted))
              });

Disclaimer: made mostly with codex...but I support it.

[Copilot]

Pull request overview

Adds OpAMP “custom messages” support to the opamp-client by letting clients advertise supported custom capabilities in AgentToServer.custom_capabilities, and by surfacing ServerToAgent.custom_message through the existing onMessage callback payload.

Changes:

• Introduces builder support for reporting custom capabilities (addCustomCapability) and plumbs this through client state and request field mapping.
• Extends MessageData and response handling so ServerToAgent.custom_message triggers Callbacks.onMessage(...) with a populated customMessage.
• Adds/updates tests to validate custom capabilities reporting and custom message callback behavior.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
opamp-client/src/test/java/io/opentelemetry/opamp/client/internal/impl/recipe/AgentToServerAppendersTest.java	Updates appender mapping test to include CUSTOM_CAPABILITIES.
opamp-client/src/test/java/io/opentelemetry/opamp/client/internal/impl/OpampClientStateTest.java	Updates state test wiring to include State.CustomCapabilities.
opamp-client/src/test/java/io/opentelemetry/opamp/client/internal/impl/OpampClientImplTest.java	Adds tests for sending custom capabilities and receiving custom messages.
opamp-client/src/main/java/io/opentelemetry/opamp/client/OpampClientBuilder.java	Adds addCustomCapability(...) and includes custom capabilities in initial state.
opamp-client/src/main/java/io/opentelemetry/opamp/client/internal/state/State.java	Adds State.CustomCapabilities state holder with new Field type.
opamp-client/src/main/java/io/opentelemetry/opamp/client/internal/response/MessageData.java	Extends callback payload to include CustomMessage.
opamp-client/src/main/java/io/opentelemetry/opamp/client/internal/request/Field.java	Adds CUSTOM_CAPABILITIES request field identifier.
opamp-client/src/main/java/io/opentelemetry/opamp/client/internal/impl/recipe/AgentToServerAppenders.java	Wires a new appender slot for CUSTOM_CAPABILITIES.
opamp-client/src/main/java/io/opentelemetry/opamp/client/internal/impl/OpampClientState.java	Adds custom capabilities to the aggregated client state.
opamp-client/src/main/java/io/opentelemetry/opamp/client/internal/impl/OpampClientImpl.java	Adds compression/wiring for custom capabilities and handles custom_message in responses.

Comments suppressed due to low confidence (1)

opamp-client/src/main/java/io/opentelemetry/opamp/client/internal/response/MessageData.java:17

• The class-level Javadoc says this message data is for "status changes", but MessageData now also carries custom messages. Updating the wording would make the API clearer for callback implementers.

/**
 * Data class provided in {@link OpampClient.Callbacks#onMessage(OpampClient, MessageData)} with
 * Server's provided status changes.
 */

[LikeTheSalad]

Thank you! 🙏 just a minor comment, not a blocker. Also, thanks for the example of how to use it.

[jackshirazi]

some minor comments