Offload calls to ServiceDiscovererFilter

Since we were not adding filters before for `ServiceDiscoverer` and we assumed that we will write `ServiceDiscoverer` and `LoadBalancer` (consume of SD stream) implementations, we chose to not offload the `Publisher` returned from `discover()`. Now with filters there is a higher possibility for user code being added in this path and hence a potential to block the eventloop. So, we should be adding offloading for these calls.

Start by tracing ServiceDiscovererFilter and the Publisher returned by discover(), focusing on where filter and user code can run on the event loop. Confirm the intended offloading behavior and verify that calls through filters no longer block the event loop.

Written by the indexing model from the issue text.