sentry_value_t custom_attrs = sentry_value_new_object();
sentry_value_set_by_key(custom_attrs, "foo", sentry_value_new_attribute(sentry_value_new_string("bar"), NULL));

sentry_value_incref(custom_attrs);
sentry_log_info("Hello", custom_attrs);

// should not contain `os.xxx`, `sentry.xxx` etc.
// {"foo":{...},"os.name":{...},"os.version":{...},"sentry.environment":{...},...}
fprintf(stderr, "%s\n", sentry_value_to_json(custom_attrs));