[release/7.0] Fix performance regression in STJ JsonNode instantiations

[github-actions]

Backport of #78130 to release/7.0

/cc @eiriktsarpalis@stephentoub

Customer Impact

Fixes a customer reported performance regression when creating JsonNode instances. This was caused by a static field in the class not having included the static keyword, resulting in expensive initializations on every JsonNode instantiation. According to benchmarks, this has resulted in said instantiation being 7x slower compared to .NET 6.

Testing

Added benchmarks to dotnet/performance measuring performance of JsonNode.

Risk

Low. Adds a missing static keyword to a readonly field declaration.

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Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
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Issue Details

---

Backport of #78130 to release/7.0

/cc @eiriktsarpalis@stephentoub

Customer Impact

Testing

Risk

IMPORTANT: Is this backport for a servicing release? If so and this change touches code that ships in a NuGet package, please make certain that you have added any necessary package authoring and gotten it explicitly reviewed.

Author:	github-actions[bot]
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-

[eiriktsarpalis]

@carlossanlop do we need to increment the servicing version for this change? It's already been incremented as part of #77388.

[ericstj]

Is there any risk that this is now static? Are they mutable and would changes made by one instance now effect others when they didn't before?

[eiriktsarpalis]

No such risk. The value is used as the default JsonSerializerOptions parameter in the JsonNode.WriteTo method -- it is passed to the built-in converters used when serializing JsonNode instances and is never passed to user-defined converters.

[ericstj]

I see. Perhaps in main we can MakeReadonly to avoid any possible changes?

[eiriktsarpalis]

Yes we could definitely make that change to avoid potential leaks of the value in the future.

[eiriktsarpalis]

Servicing approved over email.

[carlossanlop]

Now we only need a code review sign-off. @eiriktsarpalis or @layomia or @ericstj.

[carlossanlop]

Approved by Tactics and signed-off by area owner.
The CI failure is known, it's a test project having trouble loading System.Text.Json correctly. But it's pre-existing and unrelated to this PR: #77988
This is an OOB package but we already merged another backport for System.Text.Json into release/7.0, so the OOB authoring changes needed are already merged to build it in 7.0.1:

runtime/src/libraries/System.Text.Json/src/System.Text.Json.csproj

Lines 10 to 12 in 9e7c283

	<IsPackable>true</IsPackable>
	<GeneratePackageOnBuild>true</GeneratePackageOnBuild>
	<ServicingVersion>1</ServicingVersion>

Ready to merge.