NEST mapping of Dictionary<string,object>

Im trying to use NEST and can´t figure out how to use it together with this class

public class Metric {   
    public DateTime Timestamp { get; set; }     
    public Dictionary<string,object> Measurement { get; set; }  
}

How do i use the new fluent mapping with a class like this?

Im planning to use i like this:

var mesurements = new Dictionary<string, object>();
mesurements["visits"] = 1;
mesurements["url"] = new string[] {"/help", "/about"};

connection.Index(new Metric() {
      Timestamp = DateTime.UtcNow, 
      Measurement = mesurements
});

Will it be possible to write a query against the dictionary? If I wanted to get all Metrics from yesterday with a mesurenemt with a key name "visits", how will that look like ?