Michael J Swart shows how you can create a Perfmon counter which tracks a user-defined value in SQL Server:
Use this to monitor something that’s not already exposed as a performance counter. Like the progress of a custom task or whatever. If you can write a quick query, you can expose it to a counter that can be plotted by Performance Monitor.
This might track queue length, number of items processed, or whatever other time series measure you need but don’t have a UI to display.