Repository: go
Issue: x/net/http2: allow setting MaxConcurrentStreams on client
Labels: NeedsInvestigation

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Please answer these questions before submitting your issue. Thanks!
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### What version of Go are you using (`go version`)?

<pre>
$ go version
go version go1.19.13 linux/amd64
</pre>

### Does this issue reproduce with the latest release?
yes


### What operating system and processor architecture are you using (`go env`)?

<details><summary><code>go env</code> Output</summary><br><pre>
$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOENV="/root/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/root/go/pkg/mod"
GOOS="linux"
GOPATH="/root/go"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.19.13"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build2198329798=/tmp/go-build -gno-record-gcc-switches"
</pre></details>

### What did you do?
I downloaded 9M files using 100 goroutines from http2.transport.
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If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on go.dev/play is best.
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### What did you expect to see?
There are currently more than 100 active streams in http2. I thought the connection would be newly opened, but it didn't.


### What did you see instead?
After checking the http2.transport connection pool, we are reusing a single connection.
The netstat -nap check shows that there is 1 connection, rx full, tx full a lot.

The following modifications improve performance by creating connections in units of 20 active streams.
```diff
func (cc *ClientConn) State() ClientConnState {
	cc.wmu.Lock()
	maxConcurrent := cc.maxConcurrentStreams
	if !cc.seenSettings {
		maxConcurrent = 0
	}
	cc.wmu.Unlock()

+	// cc.mu.Lock()
+	// defer cc.mu.Unlock()
	return ClientConnState{
		Closed:               cc.closed,
		Closing:              cc.closing || cc.singleUse || cc.doNotReuse || cc.goAway != nil,
		StreamsActive:        len(cc.streams),
		StreamsReserved:      cc.streamsReserved,
		StreamsPending:       cc.pendingRequests,
		LastIdle:             cc.lastIdle,
		MaxConcurrentStreams: maxConcurrent,
	}
}
func (cc *ClientConn) idleStateLocked() (st clientConnIdleState) {
	if cc.singleUse && cc.nextStreamID > 1 {
		return
	}
	var maxConcurrentOkay bool
	if cc.t.StrictMaxConcurrentStreams {
		// We'll tell the caller we can take a new request to
		// prevent the caller from dialing a new TCP
		// connection, but then we'll block later before
		// writing it.
		maxConcurrentOkay = true
	} else {
		maxConcurrentOkay = int64(len(cc.streams)+cc.streamsReserved+1) <= int64(cc.maxConcurrentStreams)
	}

	st.canTakeNewRequest = cc.goAway == nil && !cc.closed && !cc.closing && maxConcurrentOkay &&
		!cc.doNotReuse &&
		int64(cc.nextStreamID)+2*int64(cc.pendingRequests) < math.MaxInt32 &&
		!cc.tooIdleLocked()

+	if st.canTakeNewRequest {
+		if cc.State().StreamsActive > 20 {
+			st.canTakeNewRequest = false
+		}
+	}
	return
}
```
As the number of active streams increases, http2 becomes slower and slower.