TUN-8861: Rename Session Limiter to Flow Limiter

## Summary
Session is the concept used for UDP flows. Therefore, to make
the session limiter ambiguous for both TCP and UDP, this commit
renames it to flow limiter.

Closes TUN-8861

proxy/proxy.go

@@ -14,8 +14,8 @@ import (
 	"go.opentelemetry.io/otel/attribute"
 	"go.opentelemetry.io/otel/trace"
 
+	cfdflow "github.com/cloudflare/cloudflared/flow"
 	"github.com/cloudflare/cloudflared/management"
-	cfdsession "github.com/cloudflare/cloudflared/session"
 
 	"github.com/cloudflare/cloudflared/carrier"
 	"github.com/cloudflare/cloudflared/cfio"
@@ -34,11 +34,11 @@ const (
 
 // Proxy represents a means to Proxy between cloudflared and the origin services.
 type Proxy struct {
-	ingressRules   ingress.Ingress
-	warpRouting    *ingress.WarpRoutingService
-	tags           []pogs.Tag
-	sessionLimiter cfdsession.Limiter
-	log            *zerolog.Logger
+	ingressRules ingress.Ingress
+	warpRouting  *ingress.WarpRoutingService
+	tags         []pogs.Tag
+	flowLimiter  cfdflow.Limiter
+	log          *zerolog.Logger
 }
 
 // NewOriginProxy returns a new instance of the Proxy struct.
@@ -46,15 +46,15 @@ func NewOriginProxy(
 	ingressRules ingress.Ingress,
 	warpRouting ingress.WarpRoutingConfig,
 	tags []pogs.Tag,
-	sessionLimiter cfdsession.Limiter,
+	flowLimiter cfdflow.Limiter,
 	writeTimeout time.Duration,
 	log *zerolog.Logger,
 ) *Proxy {
 	proxy := &Proxy{
-		ingressRules:   ingressRules,
-		tags:           tags,
-		sessionLimiter: sessionLimiter,
-		log:            log,
+		ingressRules: ingressRules,
+		tags:         tags,
+		flowLimiter:  flowLimiter,
+		log:          log,
 	}
 
 	proxy.warpRouting = ingress.NewWarpRoutingService(warpRouting, writeTimeout)
@@ -160,12 +160,12 @@ func (p *Proxy) ProxyTCP(
 
 	logger := newTCPLogger(p.log, req)
 
-	// Try to start a new session
-	if err := p.sessionLimiter.Acquire(management.TCP.String()); err != nil {
-		logger.Warn().Msg("Too many concurrent sessions being handled, rejecting tcp proxy")
-		return pkgerrors.Wrap(err, "failed to start tcp session due to rate limiting")
+	// Try to start a new flow
+	if err := p.flowLimiter.Acquire(management.TCP.String()); err != nil {
+		logger.Warn().Msg("Too many concurrent flows being handled, rejecting tcp proxy")
+		return pkgerrors.Wrap(err, "failed to start tcp flow due to rate limiting")
 	}
-	defer p.sessionLimiter.Release()
+	defer p.flowLimiter.Release()
 
 	serveCtx, cancel := context.WithCancel(ctx)
 	defer cancel()