Store Metric Aggregation metadata into a field rather than properties dictionary to improve performance

[cijothomas]

AutoCollected MetricAggregator is configured as a TelemetryProcessor to be run before Sampling processors, and every item processed by it is tagged with a custom property indicating the version of the Aggregator. Accessing the Properties collection is expensive as its implemented as a ConcurrentDictionary.
https://github.com/Microsoft/ApplicationInsights-dotnet/blob/develop/src/Microsoft.ApplicationInsights/Metrics/Extensibility/AutocollectedMetricsExtraction/AutocollectedMetricsExtractor.cs#L216

Proposal here is to create a non-public field for DependencyTelemetry and RequestTelemetry to store this meta data. So MetricAggregator will use this field, avoiding expensive Dictionary access. To avoid change in behaviour, Getter for Properties will copy the value from the internal field to the Properties Dictionary. (during serialization properties are accessed and the field is copied over to the Dictionary, but this is happens only for those items which are sampled in)

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