Runtime subscription model with Azure Functions

Question

I would like an Azure Function to be able to subscribe to certain events with a custom filter at runtime. Think of it like this: a separate HTTP trigger results in the function needing to subscribe to future events for a given customer ID only.

What is the best way of achieving this? I have complete freedom over the event infrastructure.

• I like the look of Azure Web PubSub, but I don't see a way of adding the function's PubSub connection to a group at runtime to only receive messages from a group only.
• I could just subscribe to all PubSub events then filter the events as the first step of the function, but that would mean a tonne of wasted invocations.
• I could use Event Grid and create a subscription at runtime, but that feels wrong as it means the function would need to know about itself (i.e. where EventGrid should send events to), and also the limit of 500 subscriptions per topic would become an issue as things scale.

Or am I misusing what Azure Functions are meant for and I should have some other infrastructure doing this instead?

This seems like a simple ask, so either I'm missing something, or there's a solution available that I haven't seen.