Recent changes to EventSource startup caused the IL trimmer to include portions of the DiagnosticSource assembly. Adding the IsMeterSupported feature check wasn't sufficient because EventSource also roots all of its methods via a DynamicallyAccessedMembers attribute. To ensure that the new methods can be removed when needed I refactored them into a separate EventSourceInitHelper class that won't be rooted by the existing DAM attribute.

When EventSouce.IsSupported is false I'd expect the entire EventSourceInitHelper class to be unreachable. If only EventSource.IsMeterSupported is false then I'd expect PreregisterEventProviders and GetMetricsEventSource() to be unreachable but the rest of the class will remain.

Hopefully this really fixes #106265 this time.