Hello Ken,
Using client library it is not possible to get the output in JSON format. Alternaltively you an use the CURL operation to get the output in JSON format. Example JSON query is give below. Please give it a try and let me know if you have any further questions.
CURL example:
curl -H "Authorization: Bearer
ACCESS_TOKEN_HERE"
-H "developer-token:
DEVELOPER_TOKEN_HERE"
-H "login-customer-id:
LOG_IN_CUSTOMER_ID"
-H "Content-Type: application/json"
https://googleads.googleapis.com/v1/customers/CLIENT_CUSTOMER_ID_HERE/googleAds:search --data '{query: "SELECT
campaign.id,
campaign.name, metrics.clicks, metrics.impressions, metrics.conversions, metrics.cost_micros FROM campaign WHERE segments.date DURING LAST_30_DAYS ORDER BY
campaign.id"}'
Regards,
Sai Teja, Google Ads API Team

ref:_00D1U1174p._5001UCZwK3:ref