Fix agent lifecycle events: Start instance name and Shutdown delivery

[wu-sheng]

Fix agent lifecycle events: the Start event missed the service instance name, and the Shutdown event was never delivered

• Add a unit test to verify that the fix works.
• Explain briefly why the bug exists and how to fix it.

Why the bug exists

BootService#priority() is documented as "BootServices with higher priorities will be started earlier, and shut down later", but ServiceManager sorted the phases the opposite way — prepare()/startup() ascending and shutdown() descending. So the Integer.MAX_VALUE services ran last on boot and first on shutdown, breaking two lifecycle events:

• ServiceInstanceGenerator (MAX) prepared last, so EventReportServiceClient built the Start event before INSTANCE_NAME was generated → empty serviceInstance.
• GRPCChannelManager (MAX) shut down first, closing the gRPC channel before EventReportServiceClient.shutdown() could send the Shutdown event. A second defect compounded it: the event stub's deadline was fixed once at connect time (withDeadlineAfter), so for any JVM alive longer than the upstream timeout the Shutdown RPC failed with DEADLINE_EXCEEDED even on a live channel.

How it's fixed

• ServiceManager: sort prepare()/startup() descending and shutdown() ascending, matching the documented contract (foundational services come up first and go down last).
• EventReportServiceClient: align with the stable TraceSegmentServiceClient / ServiceManagementClient idiom — volatile status/stub, a plain stub built on CONNECTED before the status is published, and a fresh deadline applied per send. The Start-event gate now uses explicit readiness (bootCompleted + CONNECTED), with reported meaning sent/in-flight, so a failed send retries on the next connect instead of being dropped (this also fixes the previously inverted compareAndSet gate).
• KafkaProducerManager: no longer derives its priority from GRPCChannelManager — that coupling flipped under the corrected order and would close the shared producer before the Kafka reporters stop. It now uses a constant priority higher than the default-priority Kafka reporters, so the producer is closed only after they stop sending.

Verification

Added ServiceManagerOrderingTest (pins boot descending / shutdown ascending) and a KafkaProducerManager priority test; apm-agent-core and kafka-reporter-plugin suites pass with 0 checkstyle violations. Also verified end-to-end against a local OAP + BanyanDB: a service instrumented with the built agent reports a Start event carrying the instance name and, on graceful shutdown (including JVMs running well past the upstream timeout), a Shutdown event.

• Update the CHANGES log.