[litianningdatadog]

A temp to address the data loss due to a fundamental mismatch between synchronous signals and asynchronous effects

How It Works:

The new shutdown sequence ensures messages are not lost by:

1. Cancelling TelemetryListener and awaiting HTTP server shutdown
2. Dropping TelemetryListener to close logs_tx channel
3. Cancelling LogsAgent and awaiting its completion (ensuring all logs are drained)
4. Sending tombstone event only after all messages are processed
5. Dropping event_bus_tx and waiting for tombstone to be processed

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Justification: instead of hoping async work completes in time, we now enforce the order:

1. Cancel TelemetryListener → await HTTP shutdown (guarantees no new messages)
2. Drop TelemetryListener → closes logs_tx (signals end of input)
3. Cancel LogsAgent → await draining completion (guarantees all messages forwarded)
4. Send tombstone → now we know all messages are processed
5. Drop event_bus_tx → safe because no more senders

Each await transforms an asynchronous cancellation into a synchronous completion guarantee.

[duncanista]

closing this as stale. #1455 makes the same fix, it drains the logs agent before sending the tombstone. feel free to reopen if still needed.