Add InitialSampleRate option to the EMADynamicSampler

[tdarwin]

Is your feature request related to a problem? Please describe.
Folks dealing with a lot of new keys tend to see a spike in real traffic volume until the adjustment interval puts the keys in the keyspace and gives those keys a sample rate.

Describe the solution you'd like
Just like with the EMAThroughputSampler, it'd be really nice to have a configurable InitialSampleRate for traces with new keys.

Describe alternatives you've considered
Not sure there is one unless you wanted this to be a global configurable