Pass the functions host name (app name) to workers on startup

OpenTelemetry says that we should be logging our function names like:

> The full name `/`

https://opentelemetry.io/docs/reference/specification/resource/semantic_conventions/faas/

However, getting the app name is fairly difficult from the worker. It depends on where we're running (windows/linux/k8s/elsewhere) and duplicating that logic from the host to every worker seems like a bad idea.

The host should pass details like this to the worker via env vars like it does with other metadata -- I recommend `FUNCTIONS_APPLICATION_NAME` or `FUNCTIONS_HOST_NAME` (which may be more appropriate).