Hey there!
If you're getting HTML back from a rest API query, then usually the URL is misconfigured or you're hitting rate limits. In this case it looks like a 404, so most likely a URL issue.
Took a quick stab at this with a test project and it seems to be working like this:
Perhaps it just needs the /properties/ in front.
N.B I'm also using OAuth 2 with https://www.googleapis.com/auth/analytics for my scopes to rule out any permissions issues.
Either way, it should just be a URL config issue.
Cheers,
Isaac