Add rate limiter, rename maxConn into inFlightReq

Co-authored-by: Julien Salleyron <julien.salleyron@gmail.com>
Co-authored-by: Jean-Baptiste Doumenjou <jb.doumenjou@gmail.com>

pkg/middlewares/inflightreq/inflight_req.go

@@ -0,0 +1,57 @@
+package inflightreq
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+
+	"github.com/containous/traefik/v2/pkg/config/dynamic"
+	"github.com/containous/traefik/v2/pkg/log"
+	"github.com/containous/traefik/v2/pkg/middlewares"
+	"github.com/containous/traefik/v2/pkg/tracing"
+	"github.com/opentracing/opentracing-go/ext"
+	"github.com/vulcand/oxy/connlimit"
+)
+
+const (
+	typeName = "InFlightReq"
+)
+
+type inFlightReq struct {
+	handler http.Handler
+	name    string
+}
+
+// New creates a max request middleware.
+func New(ctx context.Context, next http.Handler, config dynamic.InFlightReq, name string) (http.Handler, error) {
+	ctxLog := log.With(ctx, log.Str(log.MiddlewareName, name), log.Str(log.MiddlewareType, typeName))
+	log.FromContext(ctxLog).Debug("Creating middleware")
+
+	if config.SourceCriterion == nil ||
+		config.SourceCriterion.IPStrategy == nil &&
+			config.SourceCriterion.RequestHeaderName == "" && !config.SourceCriterion.RequestHost {
+		config.SourceCriterion = &dynamic.SourceCriterion{
+			RequestHost: true,
+		}
+	}
+
+	sourceMatcher, err := middlewares.GetSourceExtractor(ctxLog, config.SourceCriterion)
+	if err != nil {
+		return nil, fmt.Errorf("error creating requests limiter: %v", err)
+	}
+
+	handler, err := connlimit.New(next, sourceMatcher, config.Amount)
+	if err != nil {
+		return nil, fmt.Errorf("error creating connection limit: %v", err)
+	}
+
+	return &inFlightReq{handler: handler, name: name}, nil
+}
+
+func (i *inFlightReq) GetTracingInformation() (string, ext.SpanKindEnum) {
+	return i.name, tracing.SpanKindNoneEnum
+}
+
+func (i *inFlightReq) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
+	i.handler.ServeHTTP(rw, req)
+}