Reduce allocation cost of HttpRequestOptions

[antonfirsov]

R9 is using HttpRequestMessage.Properties/Options in several cases and we have plans in #86281 to utilize it further. The property bag contains very few elements typically, so the internal dictionary can be substituted by linear lookup over a list of elements.

My recommendation would be to embed a few key-value pairs and allocate an Array over a threshold, like we do in TagList.

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Issue Details

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R9 is using HttpRequestMessage.Properties/Options in several cases and we have plans in #86281 to utilize it further. The property bag contains very few elements typically, so the internal dictionary can be substituted by linear lookup over a list of elements.

My recommendation would be to embed a few key-value pairs and allocate an Array over a threshold, like we do in TagList.

Author:	antonfirsov
Assignees:	-
Labels:	area-System.Net.Http
Milestone:	Future

[antonfirsov]

/cc @noahfalk in case it makes sense to add this to some R9 project board for tracking.