Fix an issue where we are not injecting more HTTP2 connections when we should

[geoffkizer]

Fixes#55980

This is a regression caused by my connection pooling changes.

For HTTP2, we constrain our connection injection logic to never inject more than one connection at a time. On successful connection establishment, we should check if we need to create yet another connection to handle the queued requests. Unfortunately, we aren't doing this today.

The specific problem is a scenario like this:

(1) User submits one request. We enqueue it and kick off an HTTP2 connection attempt.
(2) User submits 100 more requests. There are now 101 requests in the queue.
(3) Connection attempt succeeds. We dequeue 100 requests and give them to the new connection to be processed.
(4) There's still one request in the queue, so we should be kicking off another connection attempt -- but currently we don't.

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Issue Details

---

Fixes #55980

This is a regression caused by my connection pooling changes.

For HTTP2, we constrain our connection injection logic to never inject more than one connection at a time. On successful connection establishment, we should check if we need to create yet another connection to handle the queued requests. Unfortunately, we aren't doing this today.

The specific problem is a scenario like this:

(1) User submits one request. We enqueue it and kick off an HTTP2 connection attempt.
(2) User submits 100 more requests. There are now 101 requests in the queue.
(3) Connection attempt succeeds. We dequeue 100 requests and give them to the new connection to be processed.
(4) There's still one request in the queue, so we should be kicking off another connection attempt -- but currently we don't.

Author:	geoffkizer
Assignees:	-
Labels:	area-System.Net.Http
Milestone:	-

[geoffkizer]

/azp run runtime-libraries-coreclr outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[geoffkizer]

/azp run runtime-libraries-coreclr outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[stephentoub]

The outerloop System.Buffers failures will be fixed by #56068. Sorry about that.

[geoffkizer]

No worries. I doubt it will be the only outerloop failure :)

[wfurt]

LGTM