Use TryAdd() or GetOrAdd() instead of TryGetValue()+Add()

[scale-tone]

https://github.com/Azure-Samples/service-fabric-dotnet-data-aggregation/blob/master/HealthMetrics.CountyService/Controllers/CountyHealthController.cs#L97

There's a dedicated TryAddAsync() method for that, why not using it?

[masnider]

This sample actually predated that method, but you're right it could be easily substituted here and in other places where this pattern was used. I think we did a pass at some point to replace all these, so this one was probably missed. PR it! Should be pretty straighforward replacement if you're willing.