Avoid calling TelemetryClient.Initialize() explicitly to set instrumentation keys

[cijothomas]

Once this issue is fixed in base sdk (#966), modify RequestCollector telemetry module to use the new API, to improve perf and avoid customer confusion.

[cijothomas]

This is done for Request collection module - Initializers are called only once.
For dependency collector, initializers are still called more than once. Need to see if that can be improved as well. I'll keep the issue open as its partially addressed only.

[github-actions]

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