fix: restrict to a choice of a list of values the --target option in api command

[smehboub]

Hello,

When we execute :

slo-generator api -c config.yaml --target test

We get the error :

Traceback (most recent call last):
  File "/home/test/.local/bin/slo-generator", line 8, in <module>
    sys.exit(main())
  File "/home/test/.local/lib/python3.7/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/home/test/.local/lib/python3.7/site-packages/click/core.py", line 1055, in main
    rv = self.invoke(ctx)
  File "/home/test/.local/lib/python3.7/site-packages/click/core.py", line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/test/.local/lib/python3.7/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/test/.local/lib/python3.7/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
  File "/home/test/.local/lib/python3.7/site-packages/click/decorators.py", line 26, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/home/test/.local/lib/python3.7/site-packages/slo_generator/cli.py", line 182, in api
    port=port,
  File "/home/test/.local/lib/python3.7/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
  File "/home/test/.local/lib/python3.7/site-packages/functions_framework/_cli.py", line 37, in _cli
    app = create_app(target, source, signature_type)
  File "/home/test/.local/lib/python3.7/site-packages/functions_framework/__init__.py", line 293, in create_app
    function = _function_registry.get_user_function(source, source_module, target)
  File "/home/test/.local/lib/python3.7/site-packages/functions_framework/_function_registry.py", line 48, in get_user_function
    source=source, target=target, non_target_functions=non_target_functions
functions_framework.exceptions.MissingTargetException: File /home/test/.local/lib/python3.7/site-packages/slo_generator/api/main.py is expected to contain a function named 'test'. Found: 'compute', 'convert_json', 'export', 'get_exporters', 'jsonify', 'load_config', 'make_response', 'process_batch_req', 'process_req', 'run_compute', 'run_export', 'setup_logging' instead

It is better to validate the value of the --target parameter as you already do with the --signature-type parameter, here : https://github.com/google/slo-generator/blob/master/slo_generator/cli.py#L148

Thanks in advance for feedbacks.
Rgs,

[smehboub]

Hello @lvaylet

Any news please ?

Thanks in advance.
Rgs,

Sophian