Foglight shows a parameter called "mssql_free_memory" which also shows up as "Buffer Manager Mssql Free memory"
What counter is used to collect this metric?
mssql_free_memory is the memory assigned to, but not consumed by SQL Server processes.
This is run from the buffer manager related statistics metrics (this may be why you're seeing "Buffer Manager Mssql Free memory").
It comes from sysperfinfo in SQL Server 2000 and dm_os_performance_counters in later versions of SQL Server.)