monitoring Microsoft Entra with OnPrem SolarWinds?

So as far as I can tell Microsoft doesnt appear to expose an API with anything interesting in terms of performance or availability monitoring.  It just gives you admin and reporting functionality.  So if it was me I would just start out by using SAM to monitor my ability to connect to the Entra endpoints that my users depend on.  That's enough to get you an understanding of availability.
Taking it a step further if I needed more detail I would write a few powershell scripts to actually make the various requests that my users need and interact with the service and measure how long it takes to perform those actions.

The whole point of hosted SaaS products is to not have to worry about them much, but you do need gather enough information to hold your vendors to their SLA targets.

So as far as I can tell Microsoft doesnt appear to expose an API with anything interesting in terms of performance or availability monitoring.  It just gives you admin and reporting functionality.  So if it was me I would just start out by using SAM to monitor my ability to connect to the Entra endpoints that my users depend on.  That's enough to get you an understanding of availability.
Taking it a step further if I needed more detail I would write a few powershell scripts to actually make the various requests that my users need and interact with the service and measure how long it takes to perform those actions.

The whole point of hosted SaaS products is to not have to worry about them much, but you do need gather enough information to hold your vendors to their SLA targets.