MAUI 10 layout diagnostics no-consumer path is not zero-allocation

[AdamEssenmacher]

Summary
MAUI layout diagnostics appear intended to be zero-allocation when no diagnostics consumer is active. In 10.0.60, IView.Measure / IView.Arrange still allocate when System.Diagnostics.Metrics.Meter.IsSupported == true, even with no ActivityListener or MeterListener.

This can cause unnecessary allocation and GC churn during normal layout, especially in configurations where meter support is enabled, such as debug or non-optimized builds.

Observed Impact
Local probe over 10,000,000 measure/arrange operations:

Mode	10.0.60 B/op	Patched B/op
meter-on-handler	408.012	~0
meter-on-handler-addmetrics, no listener	408.012	~0

Elapsed time in affected no-listener paths also improved by roughly 77-83% after gating diagnostics, though allocation/GC churn is the main issue.

Expected
With no active MAUI diagnostics consumer, layout diagnostics should avoid tag construction, instrumentation boxing, and per-operation allocation.

Likely Cause / Fix
Layout instrumentation is entered based on RuntimeFeature.IsMeterSupported, rather than checking for actual consumers:

• ActivitySource.HasListeners()
• layout counter/histogram Enabled

A local prototype that gates on these checks restored the no-listener path to effectively 0 B/op.