NetCore remove TC.Active and fix NetCore Heartbeat

[TimothyMothra]

#1953, #1954

NetCore had a dependency on TelemetryConfiguration.Active to support DiagnosticTelemetryModule and Heartbeats.
This change removes that dependency and introduces a feature flag to support TelemetryConfiguration.Active.

Changes

• remove TelemetryConfiguration.Active
• remove TelemetryModules.Instance
• Refactoring Heartbeats for NetCore scenarios previously dependent on TC.Active

Checklist

• I ran Unit Tests locally.
• CHANGELOG.md updated with one line description of the fix, and a link to the original issue if available.

For significant contributions please make sure you have completed the following items:

• Design discussion issue #
• Changes in public surface reviewed

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[cijothomas]

Lets try to do this in steps
1st remove .Active (with a config option to enable it for some customers who use it)
Add all modules including DiagModule via DI.
Fix heartbeat modules like AppServiceMetadata, AzureMetadata etc.

[TimothyMothra]

@cijothomas i don't think that will work because I think these things are too tightly coupled. Please respond if I'm misunderstanding...

• First, supporting both .Active and not would be some additional work that I don't know if it's worth the effort.
• Second, I can't support the "not .Active" config without first fixing the DiagModule and Heartbeat Modules.
• Lastly, after I've fixed all the modules, I don't think there's any value in continuing to support .Active.

[cijothomas]

@cijothomas i don't think that will work because I think these things are too tightly coupled. Please respond if I'm misunderstanding...

• First, supporting both .Active and not would be some additional work that I don't know if it's worth the effort.
• Second, I can't support the "not .Active" config without first fixing the DiagModule and Heartbeat Modules.
• Lastly, after I've fixed all the modules, I don't think there's any value in continuing to support .Active.

If there are users somehow tied to use .Active, then we want to have a way to support them. For majority of cases, we can ask to switch to DI model, but there are cases like some logging adapters for which there is no better alternate than TC.Active.

[cijothomas]

Looks good. Couple of small issues added in comment.
Also a good description of the change as a Github issue for customers.