I'm making a mobile app where the app will call a simple URL driving API The Google Directions API, the API usage limitations say it can do up to 2500 requests per day. It's on iOS. This sort of API does not require my API key or anything so I don't see how the 2500 request limit applies to this type of API unless it's the same machine making all the requests.
Could google also be tracking a part of the html header that is unique among different instances of the app regardless of IP or machine?
I hope this makes sense.
Obviously this can't be answered conclusively by someone outside of Google, but typically if there's no API key then the limit is per IP address. There isn't really a reliable "html header that is unique among different instances of the app" (I assume you mean HTTP header) - unless you set some header yourself, I really doubt that iOS injects some "app ID" into every HTTP request. But you should really contact Google with this question (do they mean 2500 per day per device or per day per app), it may be more of a legal issue than a technical one (e.g. if your app becomes popular they may have a problem with you violating the terms of service, even if they have no technical means of detecting it).