[Breaking change] Changing EventSource OnEventCommand to be consistent

[davmason]

PR #82970 was a fix for issue #61353, it included a minor breaking change in that we make the callback for EventSource.OnEventCommand after we mark all events as disabled.

Previous behavior
Prior to this change, we would issue the EventSource.OnEventCommand callback for a EventCommand.Disable prior to setting m_eventSourceEnabled=false. This means that IsEnabled would return true in the OnEventCommand callback for a user EventSource, even if the command would lead to the EventSource being disabled.

The callback happens after we turn off the ability to dispatch events though, so even if an EventSource tries to fire an event it will not be written. Also, for EventCommand.Enable we issue a consistent view - IsEnabled accurately reports the state of the world, and the EventSource can write events from the OnEventCommand callback. This change makes the EventCommand.Disable behavior match the Enable behavior.

New behavior
The change makes the callbacks consistent, now we will mark the EventSource as disabled before the callback.

@PriyaPurkayastha - We have multiple first party requests to backport this fix to 6.0 and 7.0, I would appreciate guidance on how to proceed with the backports. Is documentation sufficient?

[ghost]

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

1. Create and link to this issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.

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

[davmason]

CC @dotnet/dotnet-diag @jeffschwMSFT

[davmason]

Actually CC @dotnet/dotnet-diag (typo the first time)

[tommcdon]

Breaking change doc issues created:

• [Breaking change]: 6.0 servicing change to EventSource callback behavior docs#35313
• [Breaking change]: 7.0 servicing change to EventSource callback behavior docs#35314

[davmason]

Closing this since we have the breaking change doc bugs created