pkg/web: closure-style middlewares (#51238)

* pkg/web: closure-style middlewares

Switches the middleware execution model from web.Handlers in a slice to
web.Middleware.
Middlewares are temporarily kept in a slice to preserve ordering, but
prior to execution they are applied, forming a giant call-stack, giving
granular control over the execution flow.

* pkg/middleware: adapt to web.Middleware

* pkg/middleware/recovery: use c.Req over req

c.Req gets updated by future handlers, while req stays static.

The current recovery implementation needs this newer information

* pkg/web: correct middleware ordering

* pkg/webtest: adapt middleware

* pkg/web/hack: set w and r onto web.Context

By adopting std middlewares, it may happen they invoke next(w,r) without
putting their modified w,r into the web.Context, leading old-style
handlers to operate on outdated fields.

pkg/web now takes care of this

* pkg/middleware: selectively use future context

* pkg/web: accept closure-style on Use()

* webtest: Middleware testing

adds a utility function to web/webtest to obtain a http.ResponseWriter,
http.Request and http.Handler the same as a middleware that runs would receive

* *: cleanup

* pkg/web: don't wrap Middleware from Router

* pkg/web: require chain to write response

* *: remove temp files

* webtest: don't require chain write

* *: cleanup

pkg/tests/api/alerting/testing.go

@@ -50,10 +50,10 @@ func getRequest(t *testing.T, url string, expStatusCode int) *http.Response {
 	t.Helper()
 	// nolint:gosec
 	resp, err := http.Get(url)
+	require.NoError(t, err)
 	t.Cleanup(func() {
 		require.NoError(t, resp.Body.Close())
 	})
-	require.NoError(t, err)
 	if expStatusCode != resp.StatusCode {
 		b, err := ioutil.ReadAll(resp.Body)
 		require.NoError(t, err)