Let a handler's flush reach the client through the access log (closes #191)

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

With metrics on, adding Unwrap alone is not enough: go-http-metrics'
writer only passes a flush on when the writer inside it has a Flush
method, and it sat inside Logging, so the flush silently did nothing.
Metrics is now registered outside Logging, and the README's middleware
list follows.

A new test flushes through the production router, with the defaults
and with metrics and Sentry on, on a global route and inside 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;
+// TestFlushThroughProductionRouter in internal/server covers the
+// shipped chain.
 func TestRecovererKeepsResponseControllerWorking(t *testing.T) {
 	t.Parallel()