Fix EnableTracing option conflict with TracesSampleRate

[mattjohnsonpint]

Fixes #2366 by having TracesSampleRate and EnableTracing properties have simple get/set implementations (except for some existing validation). This issue was mainly seen during configuration binding in ASP.NET Core applications, and then - only in the case where EnableTracing was set true and TracesSampleRate was unset.

Unfortunately, the only way to implement the correct fix was to make TracesSampleRate a nullable double?. Previously, it was a non-nullable double. (The default value is now null instead of 0.0.) Normally I'd consider that to be a breaking change that could not be done without a major version bump. But in this case, it's unlikely to affect anyone, because this property is typical set, but never retrieved externally.

There are no behavioral changes. Both values work as they were originally intended.

I also improved the xmldocs, and fixed the exception type thrown when an invalid sample rate is set. (ArgumentOutOfRangeException, is more appropriate than InvalidOperationException for that).