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. SWWAF_METRICS_TOP_N
bounds the series by country, the rest counted as other.

Judgement call: a request answered at smallwebwaf's own endpoints is
neither forwarded nor refused in the client's history.
Deviation: go.mod and go.sum written by hand from the module proxy and
sum.golang.org, 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/lookup/lookup_test.go

@@ -14,6 +14,7 @@ import (
 	"time"
 
 	"sneak.berlin/go/smallwebwaf/internal/lookup"
+	"sneak.berlin/go/smallwebwaf/internal/metrics"
 )
 
 const (
@@ -194,6 +195,7 @@ func TestFailureIsLoggedWithoutTheAddressesAskedAbout(t *testing.T) {
 			URL:        lookup.URL,
 			Now:        time.Now,
 			ProcessLog: slog.New(slog.NewTextHandler(&log, nil)),
+			Metrics:    metrics.New(1),
 		})
 		g.SetTransport(geojs)
 
@@ -493,6 +495,7 @@ func start() (*standIn, *testClock, *lookup.GeoJS) {
 		URL:        lookup.URL,
 		Now:        clock.Now,
 		ProcessLog: slog.New(slog.DiscardHandler),
+		Metrics:    metrics.New(1),
 	})
 	g.SetTransport(geojs)