Fix Http.Json serialization performance by using static options

[jozkee]

Fixes #34440

There was a considerable performance regression compared against Microsoft.AspNetCore.Blazor.HttpClient caused by always instantiating a JsonSerializerOptions object when not provided, which means that System.Net.Http.Json was not actively using the caching mechanism of the JsonSerializer for types and converters.

I got the following results using the aspnet/Benchmarks repo to measure the amount of requests made to a simple WeatherForecast app that returns a List<WeatherForecast>.

The scenario keeps a thread sending Get requests for the specified duration (15s) after a specified warm-up time lapse (15s).

The benchmarks used can be found in https://github.com/Jozkee/HttpJsonBenchmarks.

I ran the scenarios 5 times each in order to get a larger sample.

I also ensured that the same version of System.Text.Json was being used (4.7.1 latest stable).

Results

Microsoft.AspNetCore.Blazor.HttpClient Version 3.2.0-preview3.20168.3
[client] 55458 iterations in 15s
[client] 53794 iterations in 15s
[client] 52779 iterations in 15s
[client] 53429 iterations in 15s
[client] 51582 iterations in 15s

System.Net.Http.Json Version 3.2.0-preview3.20175.8 Before fix
[client] 13159 iterations in 15s
[client] 12441 iterations in 15s

System.Net.Http.Json 3.2.0-dev (Private build) After fix
[client] 55352 iterations in 15s
[client] 51981 iterations in 15s
[client] 53777 iterations in 15s
[client] 52569 iterations in 15s
[client] 52655 iterations in 15s

Note: There are further optimizations that can be made/be explored, specially on the Post/Put methods but those transcend the quick gain of using the static options for cache on serialization.

cc @ericstj @joperezr @layomia

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[CodeBlanch]

@jozkee Could we squeeze a bit more performance out of it by also switching to a field and reusing the default media type instance instead of having the lambda?

        private static readonly MediaTypeHeaderValue s_defaultMediaType
            = new MediaTypeHeaderValue(JsonMediaType) { CharSet = "utf-8" };

[jozkee]

I did consider that as well, but as per @scalablecory's, we must not re-use this, it could be modified or disposed on a request, or it can be intercepted by a DelegatingHandler.