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

The access log's response writer and the metrics middleware's writer hid the writer beneath them, so a handler's flush, hijack or write deadline set through http.ResponseController failed with "not supported" behind them. The access log's writer now has Unwrap. The metrics middleware calls the library's public Measure with a writer of our own that has Unwrap, in place of std.Handler's writer, so the middleware order and what metrics record are unchanged. A test over a real connection sets a write deadline and flushes, metrics on and off, on a global route and in an admin page group, and fails without either Unwrap.

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()