Honor DO_NOT_TRACK as well as DOTNET_CLI_TELEMETRY_OPTOUT

[fowl2]

An attempt at #3917

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[marcpopMSFT]

We will continue to support the opt out mechanism described here: .NET SDK telemetry - .NET CLI | Microsoft Docs.

[delanym]

Not invented here

[delanym]

The variable DOTNET_CLI_TELEMETRY_OPTOUT is not a standard

[kasperk81]

DO_NOT_TRACK is not the standard either

summing up #6145 to reflect what community at large want:

1. the best thing would be if tracking is made opt-in.
2. the second best option is if user is asked on the first run whether to opt-in to this data collection program.
3. the third best option is to provide a way to opt-out, for which there is existing support for DOTNET_CLI_TELEMETRY_OPTOUT flag.

DO_NOT_TRACK is just the variant of third best thing, and it is not that interesting as first two options, which will really inspire confidence and transparency.

[delanym]

Tracking will never be made opt-in because who in their right mind would go out of their way to turn it on? Its unrealistic.
Your second best option is not friendly for unattended installations. It also means that if I remove the application and install it again I have to answer the same question again.
I make the decision to allow telemetry on a system level, not on an application level. If I say yes to telemetry for dotnet, chances are I'm happy to do it for others and I don't want to have to answer the same dumb question over again for every machine, for every app, for every install. It also means if I change my mind, I can change it once! DO_NOT_TRACK is my system level telemetry policy and it costs nothing for dotnet to respect it - probably not even another line of code.

[khellang]

Hey @marcpopMSFT! 👋🏻

Would it hurt anyone to honor a standard flag in addition to the existing flag? #3917 is pretty heavily upvoted. I'd love to hear some reasoning for this 😅