Poor performance of ContentType header rendering

[kyri-petrou]

Describe the bug
Hi there 👋. I was going through some profiling data of a web-app and there was a disproportionally high % of CPU being spent on the zio.http.Header$ContentType.renderedValueAsCharSequence() method (up to 5% of the application's total CPU time)

To Reproduce
Using JMH:

package example

import org.openjdk.jmh.annotations.*
import zio.http.*

import java.util.concurrent.TimeUnit

@State(Scope.Thread)
@BenchmarkMode(Array(Mode.Throughput))
@OutputTimeUnit(TimeUnit.SECONDS)
@Warmup(iterations = 3, time = 3, timeUnit = TimeUnit.SECONDS)
@Measurement(iterations = 5, time = 3, timeUnit = TimeUnit.SECONDS)
@Fork(1)
class ContentTypeRenderingBenchmark {

  private val header = Header.ContentType(MediaType.application.json)

  @Benchmark
  def rendered(): Unit =
    for _ <- 1 to 10_000 do header.renderedValue

  @Benchmark
  def interpolation(): Unit =
    for _ <- 1 to 10_000 do s"${header.mediaType.mainType}/${header.mediaType.subType}"
}

Results:

[info] Benchmark                                     Mode  Cnt     Score     Error  Units
[info] ContentTypeRenderingBenchmark.interpolation  thrpt    5  7898.494 ± 270.269  ops/s
[info] ContentTypeRenderingBenchmark.rendered       thrpt    5    62.329 ±   0.772  ops/s

Expected behaviour
I understand there is some additional logic needed in rendering the content type fully (charset, boundaries), but I would expect header.renderedValue's performance to be somewhat similar to using string interpolation

Additional context
A workaround to the issue is to use .untyped on the header, as that forces the rendered value to be cached. However, that does not work in cases where the content type header is dynamically created

[jdegoes]

/bounty $150

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Attempt	Started (GMT+0)	Solution
🟢 @kyri-petrou	Jan 6, 2024, 11:31:51 PM	#2597

[kyri-petrou]

/attempt #2573

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