Settle where a run is traced, as (project_name, address).
Exactly one of the two is set. A destination passed here beats the env vars; with
neither, LANGSMITH_AGENT_ID and LANGSMITH_AGENT_ENVIRONMENT decide, else the
project from LANGSMITH_PROJECT (or default). This is the rule the SDK applies to
a root run, for servers and apps that need to know a run's destination themselves.
A project named by the caller.
An AgentAddress named by the caller, as returned by agent_address.
