Most high-traffic REST API entry points are cached in Varnish. While we do have graphite metrics for each end point in RESTBase, those will only capture cache misses, and thus won't result in an accurate picture of overall API use.
To get accurate overall API stats, it would be helpful to count web requests according to the entry points defined in the Swagger spec (doc view). Ideally, we'd drive this entirely from the spec, and avoid the need to manually update matchers. I have written code for turning URI specs to regexps in the past, and am willing to help.
See also: T142139: Top API user agents stats