core: Count client-cancelled server calls as failed in Channelz

[JoeCqupt]

Fixes #13063

Problem

ServerCallImpl only reports the end of a call to CallTracer when the
application closes it (closeInternal) or when an internal error occurs
(handleInternalError). When the client cancels, the transport calls
ServerStreamListenerImpl.closed() with a non-OK status. That path calls
onCancel() and cancels the context, but never reports the end of the call.
The application normally doesn't call close() after it sees onCancel(),
so calls_started goes up while calls_failed does not.

Fix

• Report the call as failed in the non-OK branch of
  ServerStreamListenerImpl.closedInternal().
• Send every end-of-call report (from closeInternal, handleInternalError
  and the cancellation path) through a single reportCallEnded() guarded by
  an AtomicBoolean, so each call is counted exactly once. The guard is
  needed because:
  • the application may still call close() after the client cancelled;
  • handleInternalError reports the failure and then calls
    stream.cancel(), which leads to closed() with a non-OK status;
  • the application's close() and the transport's closed() can run on
    different threads.

The OK branch of closed() is left unchanged. A closed(OK) only happens
after the server has already closed the stream, and its status is not the
RPC's actual status, so the end of the call has already been counted in
closeInternal with the real status.

Behavior changes

• If the client cancels and the application later calls close(OK), the
  call used to be counted as succeeded. It is now counted as failed,
  because the cancellation is seen first.
• Side effect of counting each call only once: a unary call closed with
  close(OK) without sending a response used to be counted as both
  failed (from handleInternalError) and succeeded (from the finally in
  closeInternal). It is now counted only as failed. This was not a
  goal of this change; it follows from the count-once guard.

Testing

New tests in ServerCallImplTest:

• callTracer_clientCancelled_reportsCallFailed: cancellation alone counts
  one failure. This test fails without the fix.
• callTracer_clientCancelledThenClosed_reportsCallFailedOnce: cancel then
  close(OK) gives failed 1, succeeded 0.
• callTracer_closedThenClientCancelled_reportsCallSucceededOnce:
  close(OK) then cancel gives succeeded 1, failed 0.

[JoeCqupt]

@kannanjgithub PTAL