fix: traceability of the middleware plugins

pkg/server/middleware/middlewares.go

@@ -357,7 +357,7 @@ func (b *Builder) buildConstructor(ctx context.Context, middlewareName string) (
 		}
 
 		middleware = func(next http.Handler) (http.Handler, error) {
-			return plug(ctx, next)
+			return newTraceablePlugin(ctx, middlewareName, plug, next)
 		}
 	}
 

pkg/server/middleware/plugins.go

@@ -1,10 +1,14 @@
 package middleware
 
 import (
+	"context"
 	"errors"
+	"net/http"
 
+	"github.com/opentracing/opentracing-go/ext"
 	"github.com/traefik/traefik/v2/pkg/config/dynamic"
 	"github.com/traefik/traefik/v2/pkg/plugins"
+	"github.com/traefik/traefik/v2/pkg/tracing"
 )
 
 // PluginsBuilder the plugin's builder interface.
@@ -31,3 +35,25 @@ func findPluginConfig(rawConfig map[string]dynamic.PluginConf) (string, map[stri
 
 	return pluginType, rawPluginConfig, nil
 }
+
+type traceablePlugin struct {
+	name string
+	h    http.Handler
+}
+
+func newTraceablePlugin(ctx context.Context, name string, plug plugins.Constructor, next http.Handler) (*traceablePlugin, error) {
+	h, err := plug(ctx, next)
+	if err != nil {
+		return nil, err
+	}
+
+	return &traceablePlugin{name: name, h: h}, nil
+}
+
+func (s *traceablePlugin) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
+	s.h.ServeHTTP(rw, req)
+}
+
+func (s *traceablePlugin) GetTracingInformation() (string, ext.SpanKindEnum) {
+	return s.name, tracing.SpanKindNoneEnum
+}