Refine the timing to raise EVENT_CLI_POST_EXECUTE event and catch SystemExit

[haroldrandom]

What this PR do:

1. make application be aware of the SystemExit exception
2. Move a final event EVENT_CLI_POST_EXECUTE to finally statement to let application be aware of the final existing event and be able to register callbacks to do something.

Originally inspired from the logging control flow of CLI __main__.py:

    exit_code = cli_main(az_cli, sys.argv[1:])

    if exit_code and exit_code != 0:
        telemetry.set_failure()
    else:
        telemetry.set_success()

    elapsed_time = timeit.default_timer() - start_time

    az_cli.logging.end_cmd_metadata_logging(exit_code)
    sys.exit(exit_code)

except KeyboardInterrupt:
    telemetry.set_user_fault('keyboard interrupt')
    sys.exit(1)
except SystemExit as ex:  # some code directly call sys.exit, this is to make sure command metadata is logged
    exit_code = ex.code if ex.code is not None else 1

    try:
        elapsed_time = timeit.default_timer() - start_time
    except NameError:
        pass

    az_cli.logging.end_cmd_metadata_logging(exit_code)
    raise ex

The step to close log and the step to open log is not peer to peer.

The timing to open and write log is when event EVENT_INVOKER_PRE_CMD_TBL_CREATE is raises which is inside execut() of AzCliCommandInvoker. But the close step is outside of that method.

I want to register the closing file logger callback when event EVENT_CLI_POST_EXECUTE is raised.
Such that, I can safely to close the opened fds without any leaks.
So, I moved the timing to raise EVENT_CLI_POST_EXECUTE to finally statement.
Actually, it should be the right thing to do to make the event machenism integrated.

With changing like that, CLI is able to do opening and closing thing in one uniform logic section which is to leverage the whole knack framework.

Test Guide:

With this patch in Azure/azure-cli, install in editable mode, and run:
pytest -x -v --log-level=WARN {TESTS}.

[jiasli]

Let's submit the PR for Azure CLI as well and review them together.

[qianwens]

[haroldrandom]

Let's submit the PR for Azure CLI as well and review them together.

Added