Avoid race condition in EventSource_ConnectionPoolAtMaxConnections_LogsRequestLeftQueue

[MihaZupan]

#50226 added asserts around ActivityIds of events.
It is expecting that IDs of Start/Stop pairs match, relying on the order of events being the same.

The scenario is such that there are 2 requests pending from the client:

• First has already sent the request and is waiting on the server's response
• Second is waiting on the queue due to the connection limit being hit (and is therefore blocked on the first one)

The server sends the response to the first request and then processes the second request as fast as possible.

The first request will receive its response and release the second request.
It is at this point that there is a race where the second request could finish and log RequestStop before the first request does.

While this is perfectly fine correctness-wise, the test will fail as it's relying on the order of Starts being the same as that of Stops.

This PR waits before processing the second request until it confirms the first request has finished.

Fixes #52530

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Issue Details

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#50226 added asserts around ActivityIds of events.
It is expecting that IDs of Start/Stop pairs to match, relying on the order of events being the same.

The scenario is such that there are 2 requests pending from the client:

• First has already sent the request and is waiting on the server's response
• Second is waiting on the queue due to the connection limit being hit (and is therefore blocked on the first one)

The server sends the response to the first request and then processes the second request as fast as possible.

The first request will receive its response and release the second request.
It is at this point that there is a race where the second request could finish and log RequestStop before the first request does.

While this is perfectly fine correctness-wise, the test will fail as it's relying on the order of Starts being the same as that of Stops.

This PR waits before processing the second request until it confirms the first request has finished.

Fixes #52530

Author:	MihaZupan
Assignees:	-
Labels:	area-System.Net.Http
Milestone:	6.0.0

[MihaZupan]

/azp run runtime-libraries-coreclr outerloop

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