Allow for changing the default NewExemplarMinInterval

[mrmartan]

...without having to replace the entire default exemplar behavoir. Currently I have to take dependency on the ExemplarBehavior..ctor and while now there are only two properties that might change and I don't think this is a good design.

Current:

Metrics.DefaultFactory.ExemplarBehavior = new ExemplarBehavior
{
    DefaultExemplarProvider = (_, _) => Exemplar.FromTraceContext(),
    NewExemplarMinInterval = TimeSpan.FromMinutes(1)
};

Expected (or something like):

Metrics.DefaultFactory.DefaultExemplarBehavior.NewExemplarMinInterval = TimeSpan.FromMinutes(1);