What was going on
They were splitting one complex Mezmo pipeline into several pipelines sharing a single HTTP shared_source. With single HTTP sources they could build the endpoint URL for their log forwarder from the pipeline ID, but the shared_source's ID is not the value that ends up in the final shared-source endpoint. Nothing in Terraform state exposed the endpoint either, so they had to create the shared source and then manually copy the endpoint URL into the log forwarder. They asked whether any Terraform attribute could give them the endpoint dynamically.
What fixed it
A Mezmo maintainer confirmed this was an oversight on their side: the ID that can be used to derive the shared-source endpoint is the consumer_id. They added that attribute in PR #214, and it shipped in provider v4.2.0. Upgrade the provider to v4.2.0 or later and read the endpoint from the new consumer_id attribute instead of copying it by hand.