Enable or disable telemetry

[epananth]

To double check:

• The right tests are in and and the right validation has happened. Guidance: https://github.com/dotnet/core-eng/tree/master/Documentation/Validation

Added ability to turn off telemetry in Telemetry.cs

was able to test this in release pipeline -> https://dnceng.visualstudio.com/internal/_build/results?buildId=1241503&view=logs&j=d7f318d2-3658-5f56-29df-16b211ec56a6&t=7851342b-b7f6-561e-0dbe-e63708dfa83e

[michellemcdaniel]

I'm happy with this, but make sure Chris gets a chance to review

[epananth]

@chcosta can you please take a look at this before I merge this change?

[chcosta]

Remove the "true" explicit check and this LGTM

[ghost]

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