Use Convert instead of cast for formatting ElapsedMilliseconds

[ixnas]

For a project I'm using SerilogWeb.Classic to log HTTP requests. The ECS formatter crashed when it tried to cast ElapsedMilliseconds from a message template, because SerilogWeb.Classic passed a long, while the ECS formatter expected a double.

Because the property is boxed, the formatter would crash when trying to cast a boxed long to a double.

I can't find any specification on the specific type that this property should have, so I figured the formatter should be agnostic about the exact numerical type.

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