Lazily Instantiate ConcurrentDictionaries to improve performance

[cijothomas]

In TelemetryContext, and in auto-generated constructors of each base telemetry item, ConcurrentDictionaries are created for each telemetry item creation. This is a perf hit as ConcurrentDictionaries are expensive to create, and in most cases, the item is sampled out, so this ConccurentDictionary allocation is wasted.

https://github.com/Microsoft/ApplicationInsights-dotnet/blob/develop/src/Microsoft.ApplicationInsights/DataContracts/TelemetryContext.cs#L42

https://github.com/Microsoft/ApplicationInsights-dotnet/blob/develop/src/Microsoft.ApplicationInsights/Extensibility/Implementation/External/AvailabilityData_types.cs#L97

Proposal is to make the ConcurrentDictionary creation lazy, and improve performance.

Version Info

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