docs: Update dictConfig snippet

[gkevinzheng]

The previous dictConfig snippet did not include the proper include logging.config statement or logging.config.dictConfig statement in the snippet as seen in the documentation to actually apply the dictConfig. The configuration also did not run properly, failing with traceback

Traceback (most recent call last):
  File "/usr/local/google/home/kevzheng/.pyenv/versions/3.12.0/lib/python3.12/logging/config.py", line 872, in add_handlers
    logger.addHandler(self.config['handlers'][h])
                      ~~~~~~~~~~~~~~~~~~~~~~~^^^
  File "/usr/local/google/home/kevzheng/.pyenv/versions/3.12.0/lib/python3.12/logging/config.py", line 344, in __getitem__
    value = dict.__getitem__(self, key)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^
KeyError: 'console'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/local/google/home/kevzheng/.pyenv/versions/3.12.0/lib/python3.12/logging/config.py", line 660, in configure
    self.configure_root(root)
  File "/usr/local/google/home/kevzheng/.pyenv/versions/3.12.0/lib/python3.12/logging/config.py", line 906, in configure_root
    self.common_logger_config(root, config, incremental)
  File "/usr/local/google/home/kevzheng/.pyenv/versions/3.12.0/lib/python3.12/logging/config.py", line 889, in common_logger_config
    self.add_handlers(logger, handlers)
  File "/usr/local/google/home/kevzheng/.pyenv/versions/3.12.0/lib/python3.12/logging/config.py", line 874, in add_handlers
    raise ValueError('Unable to add handler %r' % h) from e
ValueError: Unable to add handler 'console'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/google/home/kevzheng/.pyenv/versions/3.12.0/lib/python3.12/logging/config.py", line 912, in dictConfig
    dictConfigClass(config).configure()
  File "/usr/local/google/home/kevzheng/.pyenv/versions/3.12.0/lib/python3.12/logging/config.py", line 662, in configure
    raise ValueError('Unable to configure root '
ValueError: Unable to configure root logger

This PR adds a more realistic sample dictConfig that might be used in an actual Cloud environment, while still maintaining how to set up both CloudLoggingHandler and StructuredLogHandler in dictConfig in a Cloud environment.

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/usage_guide.py:489, tag logging_dict_config

You are about to delete 1 region tag.

• samples/snippets/usage_guide.py:491, tag logging_dict_config

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