Aside from this insight that's available in Admin > Web Services:
Anything else requires help from Support. In fact, Support themselves suggests we continue to add/vote on ideas to make this more self-serve for the customer:
Here's an example of a report that Support can provide you if you request it:
'tis for this reason that I run my API calls through an API gateway (Apigee, though you could use AWS API GW or another).
The upside to the low daily limit is that means you can afford to use a commercial API GW service, which expects millions of hits per day.
No, I normally do just that (AWS API + some logging tool a la Splunk); it's just in this specific case I need to work backwards to track down a rogue API connector that's...somewhere? Perhaps on an owned server, perhaps elsewhere? Shadow IT at its worst.