Countly web analytics.
Countly API is a free Open Source Projects API that requires no API key, is only available over HTTP and has unconfirmed CORS support.
Comprehensive API reference for Countly Enterprise
Details from api.count.ly.

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The Countly API is provided by Countly Enterprise API Documentation (api.count.ly). Comprehensive API reference for Countly Enterprise
Yes. The Countly API is listed in the Open Source Projects category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The Countly API requires no authentication, so you can start making requests without signing up.
CORS support for the Countly API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
No. The Countly API is only available over plain HTTP, which browsers block on HTTPS sites. Call it from a server instead.