Let a handler flush or set a write deadline through the access log and metrics (closes #191)

The access log's response writer had no Unwrap method, so
http.ResponseController stopped at it and a handler's Flush or
SetWriteDeadline returned http.ErrNotSupported. It now has one.

With metrics on, the metrics library's std.Handler put a writer of its
own in the chain with the same gap. The Metrics middleware now calls
the library's Measure with a writer of ours that has Unwrap, and keeps
its place in the chain.

A new test sets a write deadline and flushes through the production
router over a real connection, with metrics off and on, on a global
route and in an admin page route group.

Model: opus-5-5

internal/middleware/recoverer_test.go

@@ -627,11 +627,9 @@ func TestRecovererIgnoresANonPanickingHandler(t *testing.T) {
 // net/http's own writer from http.ResponseController, so a handler
 // that flushes or sets a deadline starts failing.
 //
-// The recoverer is the only middleware in the chain here. The access
-// logger's own wrapper does not implement Unwrap, so a chain
-// containing it fails this regardless of what the recoverer does;
-// what is being pinned is that the recoverer adds no such opacity of
-// its own.
+// The recoverer is the only middleware in the chain here;
+// TestResponseControllerThroughProductionRouter in internal/server
+// covers the shipped chain.
 func TestRecovererKeepsResponseControllerWorking(t *testing.T) {
 	t.Parallel()