[Release/6.0] Port EventCounters multi session support to 6.0

[davmason]

Ports #82970 to 6.0

Customer Impact

Currently multiple different EventCounters sessions can be started but when any session stops we stop emitting counters for all events. This is true for dotnet-counters and manually enabling counters via in an process EventListener or via ETW/EventPipe/LTTNG.

We have quite a few internal and external teams using EventCounters, so as time goes on we find that they are disabling each other's sessions more and more. We have received requests from internal and external partners that this be fixed in servicing.

Testing

Partner team validation that it fixes their scenario.

Risk

This fix includes a minor breaking change as described in #84586. We previously would issue callbacks for EventSource Disable events before we fully marked the EventSource as disabled, but after we disallowed any further events from being sent. With this change we will issue callbacks after we mark the EventSource as fully disabled.

I cannot think of a scenario this would break, but it is different behavior and could theoretically cause issues for customers.

[jeffschwMSFT]

approved. we will take for consideration in 6.0.x. please get a code review

[ghost]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.

[noahfalk]

The change looks correct as-is, though I think more than half the non-assert, non-comment portion of the change is refactoring that doesn't change the final behavior. If you've already spent a bunch of time testing and this would reset your work I wouldn't worry about it, but ideally we'd skip the refactoring and just do the minimal functional change.

[carlossanlop]

As indicated in the 7.0 PR - Code complete is May 15th for the June Release.

[davmason]

Rebasing against release/6.0-staging latest to trigger a new CI run

[cmoaciopm]

Could some please tell when will 6.0.18 be released? I saw 6.0.18 milestone was already closed. Does that mean it will be released soon? BTW, I found 6.0.17 is not released yet. Not sure whether 6.0.17 will be skipped or not. I'm asking this question because there is breaking change for the fix. Hope someone can help to clarify my question. Thanks.