Is there a combination of columns in sys.dm_exec_sessions that is unique per the server? In SQL Server, each session has its own spid. Spids are unique at any given notice, but spids, like process and thread identifiers in the OS are recycled.

However sys.dm_exec_sessions has other columns with session metadata. Is there a combination of columns that is guaranteed to be unique for a server instance?
Sessions belong to connections (1:M) and connections have unique GUID identifier, see sys.dm_exec_connections:


  connection_id uniqueidentifier Identifies each connection uniquely. Is
  not nullable.


Whenever you capture sys.dm_exec_sessions, join with sys.dm_exec_connections and capture the connection_id too.