Serve Prometheus metrics behind SWWAF_METRICS_TOKEN (closes #23)

GET /_smallwebwaf/metrics answers in the Prometheus text format for a
request carrying SWWAF_METRICS_TOKEN, 401 without it and 404 while it is
unset. Every request under /_smallwebwaf/ but the health check now goes
through the checks and is answered where it would be forwarded, 404 for
any path but the metrics, so none reaches the app. In the client's
history a 401 counts as refused, the metrics and the 404s as neither.
SWWAF_METRICS_TOP_N bounds the series by country, the rest counted as
other.

Deviation: go.mod and go.sum written by hand, as go runs only through
make.
Deviation: no metrics yet for state files read again after an edit or
edits set aside; that work is not merged.

Model: opus-5-5

internal/ratelimit/ratelimit.go

@@ -71,7 +71,9 @@ type History struct {
 	Country  string    `json:"country,omitempty"`
 	LookedUp time.Time `json:"looked_up,omitzero"`
 	// Requests are all the client's requests: Forwarded those passed to
-	// the app, Refused those refused before anything reached it.
+	// the app, Refused those refused before anything reached it, a 401 at
+	// smallwebwaf's own endpoints included, and neither the others
+	// smallwebwaf answered there.
 	Requests  int64 `json:"requests"`
 	Forwarded int64 `json:"forwarded"`
 	Refused   int64 `json:"refused"`
@@ -103,9 +105,12 @@ type Offences struct {
 type Request struct {
 	// Country is the client's country, when the request looked it up.
 	Country string
-	// Forwarded is true for a request passed to the app, false for one
-	// refused before anything reached it.
+	// Forwarded is true for a request passed to the app, Refused for one
+	// refused before anything reached it, a 401 at smallwebwaf's own
+	// endpoints included. Both are false for any other request smallwebwaf
+	// answered there.
 	Forwarded bool
+	Refused   bool
 	// Status is what the client was sent, 0 if nothing was.
 	Status int
 	// RequestBytes and ResponseBytes are the body bytes of the request
@@ -199,7 +204,9 @@ func (l *Limiter) AddToHistory(client netip.Prefix, now time.Time, r Request) {
 	h.Requests++
 	if r.Forwarded {
 		h.Forwarded++
-	} else {
+	}
+
+	if r.Refused {
 		h.Refused++
 	}
 
@@ -235,6 +242,14 @@ func (l *Limiter) Requests(netblock netip.Prefix) int64 {
 	return requests
 }
 
+// Len returns how many clients are in the table.
+func (l *Limiter) Len() int {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	return l.clients.Len()
+}
+
 // Snapshot returns every client in the table, sorted by address, as
 // clients.json lists them.
 func (l *Limiter) Snapshot() []Client {