in_monitor_agent: Add include_config parameter to remove config field from response

[repeatedly]

In our experience, almost users don't need config param because this parameter is not needed for buffer monitoring.
Removing it avoids unnecessary json processing and reduces response size.

config param itself is useful when user wants to check actual buffer configuration.
So add with_config query parameter to keep previous behaviour.

[repeatedly]

@tagomoris How about this change?

[tagomoris]

This fix changes default behavior, and there's no way to change plugin's default behavior (the only way to change output is to change fetcher - any other scripts or agents which users wrote).

I think it's better to have a new config_param to control whether output includes configuration or not without any query parameters.

[repeatedly]

Applied review and change PR title.

[tagomoris]

test_configure should have an assertion about include_config. Others look good to me.

[repeatedly]

Good point. I just added assertion. After test passed, I will merge this patch.