metrics: rate limit /metrics per client address before Basic Auth (closes #101)

/metrics is behind a password, and REPO_POLICIES.md requires rate
limiting on password logins. Each client address may now send it 30
requests a minute, counted by httprate before Basic Auth, so failed
logins use up the allowance and a request over it gets 429 without
the password being checked. The address is the one the existing
trusted-proxy logic in internal/middleware works out, with IPv6
addresses grouped by /64. A Prometheus server scraping every 15
seconds sends 4 requests a minute.

Model: opus-5-5

internal/middleware/export_test.go

@@ -0,0 +1,10 @@
+package middleware
+
+import "time"
+
+// The /metrics rate limit, exported so the tests can count requests
+// against it.
+const (
+	MetricsRequestLimit                = metricsRequestLimit
+	MetricsRequestWindow time.Duration = metricsRequestWindow
+)