Memory leak with `System.Diagnostics.Metrics.Meter` when not disposed

[hendriklhf]

Similar to #114599. When creating a Meter, it's added to a static List<Meter> and disposing it will remove it from the list, but when Dispose is never called, the Meter will stay in the list forever and never be collected.

runtime/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Metrics/Meter.cs

Lines 96 to 99 in e220a94

	lock (Instrument.SyncObject)
	{
	s_allMeters.Add(this);
	}

runtime/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Metrics/Meter.cs

Lines 504 to 515 in e220a94

	lock (Instrument.SyncObject)
	{
	if (Disposed)
	{
	return;
	}
	Disposed = true;
	s_allMeters.Remove(this);
	instruments = _instruments;
	_instruments = new List<Instrument>();
	}

Reproduction:

using System.Diagnostics.Metrics;

for (int i = 0;; i++)
{
    Meter m = new("");
    if (i % 1_000_000 == 0)
    {
        Console.WriteLine(Environment.WorkingSet.ToString("N0"));
    }
}

[KalleOlaviNiemitalo]

I'm not sure this is a bug. The application might create a Meter, and then some sort of observable Instrument with a callback, without saving either reference anywhere, and expect the Meter and Instrument to stay rooted and accessible to diagnostic tools.

Creating a million Meter instances does not seem a normal scenario anyway. I'd expect one Meter per library, and a unique documented name for each.

[hendriklhf]

Yes, that's true. It will probably never result in a noticeable memory leak in a normal application, but I would not expect any object to leak when forgetting to call Dispose 🤔

[KalleOlaviNiemitalo]

It's not really a leak if the Meter objects can still be enumerated and the values of their instruments can still be read, by tools like dotnet-metrics. IIRC, timer objects are similarly rooted until explicitly disposed/stopped. (But which of the three timer classes was that…?)

[tarekgh]

@KalleOlaviNiemitalo is correct. This is not a bug. As long as the meter object is alive, all registered instruments shouldn't be collected.