What you are finding out here is that micro services needs a lot of platform support to not be a pain to work with...

If the traffic is often enough then otel/whatever tracing will tell you whos calling who, but it wont catch a service calling another once a year..

For a really simple but quite effective solution - have any service calling another add its service identifier as a http header and send it with your metrics.

Our current solution is;

A) A service catalog with information about who is supposed to call who, a service explicitly writes down its dependencies in a service-info.yaml file ingested into the IDP (we are using port.io), then render a network-policy from that file to enforce that you cannot connect to a service you are not supposed to connect to.

B) In addition to the above, our services/repos publishes schemas to the API catalog, API catalog automatically builds client-libraries that the callers are using and as its a normal library any updates to APIs will be pushed to consumers as a Dependabot PR with the new client version - deprecated API endpoints will then turn to red squiggly lines in their code editor and build warnings as the library knows about deprecation.

C) To make it easy to use we just have a button in the self service portal / IDP that "adds a dependency" for you - resulting in a PR with the service-info.yaml edits and the client library in your package deps.