Retire IResourceMonitor set of API in favor of observable instruments

[evgenyfedorov2]

Currently, ResourceMonitoring can be consumed in two ways:

• via IResourceMonitor API which internally uses ResourceMonitorService implementing BackgroundService
• via Observable instruments (metrics).

Both ways provide the same resource monitoring data, so essentially we have duplicated functionality. We can simplify it all by deprecating the IResourceMonitor API and recommend the observable instruments as the only way forward. Namely, we can deprecate following classes (or structs):
src/Libraries/Microsoft.Extensions.Diagnostics.ResourceMonitoring/SystemResources.cs
src/Libraries/Microsoft.Extensions.Diagnostics.ResourceMonitoring/Snapshot.cs
src/Libraries/Microsoft.Extensions.Diagnostics.ResourceMonitoring/ResourceUtilization.cs
src/Libraries/Microsoft.Extensions.Diagnostics.ResourceMonitoring/ISnapshotProvider.cs
src/Libraries/Microsoft.Extensions.Diagnostics.ResourceMonitoring/IResourceMonitorBuilder.cs
src/Libraries/Microsoft.Extensions.Diagnostics.ResourceMonitoring/IResourceMonitor.cs

Pros:

• Simplified API
• Improved performance, avoiding wasting resources for the BackgroundService

Cons:

• Users of the IResourceMonitor will be blocked in case they don't use .NET Metrics

[evgenyfedorov2]

@RussKie @joperezr @geeknoid @andrey-noskov - please take a look when you have time

[geeknoid]

Regarding your "con", given that we wouldn't be deleting the API and just marking it obsolete, then nobody would be 'blocked', right?

[andrey-noskov]

yes, please!
this probably should include ResourceMonitorService, CircularBuffer, Calculator and many other internal utilities that power our IResourceMonitor implementation.

[evgenyfedorov2]

Regarding your "con", given that we wouldn't be deleting the API and just marking it obsolete, then nobody would be 'blocked', right?

Yes.

Another question - I assume we will be able to delete any Obsolete API when .NET 8 support ends, e.g. in 2026?

[geeknoid]

@joperezr Are obsoleted APIs ever removed from the code base?

[joperezr]

Yes, they can be. Typically that is done in the subsequent LTS version after being marked as obsoleted (in an LTS version too). This policy is documented here: https://learn.microsoft.com/en-us/dotnet/core/compatibility/api-removal

[RussKie]

@evgenyfedorov2: sounds good to me. We can mark these API (and anything else related) as obsolete in the .NET 9 release (i.e., the dev branch). We'll also need to create migration guides.

There isn't much time before we switch to .NET 9, so we should do this before then.