How do you attribute 1M calls if the gateway strips headers?

My own API sits behind a marketplace gateway. Every caller was supposed to send an X-FP-Source header so I could tell an n8n workflow from a browser from an MCP server. Nothing ever showed up in the logs. The gateway rebuilds the request on the way through and drops custom headers, so the tag was gone before my Lambda saw it. Fix is ugly and it works: the source rides in the JSON body as _fp_source, and the handler strips it out before validation. What proved it wasn't a 200. It was one real call and a grep in CloudWatch for the [source] line. A 200 tells you nothing about attribution. submitt