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.go

@@ -19,6 +19,7 @@ import (
 	"time"
 
 	"github.com/hashicorp/golang-lru/v2/simplelru"
+	"sneak.berlin/go/smallwebwaf/internal/metrics"
 )
 
 // URL is GeoJS's country endpoint. Asked about several addresses at once,
@@ -64,6 +65,9 @@ type Params struct {
 	Now func() time.Time
 	// ProcessLog receives GeoJS's failures.
 	ProcessLog *slog.Logger
+	// Metrics count the requests to GeoJS, those that failed, and the
+	// clients that go without an answer.
+	Metrics *metrics.Metrics
 }
 
 // GeoJS looks up clients' countries through GeoJS. At most one request
@@ -73,6 +77,7 @@ type GeoJS struct {
 	url        string
 	now        func() time.Time
 	processLog *slog.Logger
+	metrics    *metrics.Metrics
 	// httpClient follows no redirect, so that visitors' addresses go to
 	// GeoJS alone: a redirect is a failure.
 	httpClient *http.Client
@@ -121,6 +126,7 @@ func New(params Params) *GeoJS {
 		url:        params.URL,
 		now:        params.Now,
 		processLog: params.ProcessLog,
+		metrics:    params.Metrics,
 		httpClient: &http.Client{
 			CheckRedirect: func(*http.Request, []*http.Request) error {
 				return http.ErrUseLastResponse
@@ -160,6 +166,9 @@ func (g *GeoJS) Country(ctx context.Context, client netip.Prefix) string {
 	defer g.mu.Unlock()
 
 	country, found := g.kept(client)
+	if !found {
+		g.metrics.GeoJSUnanswered.Inc()
+	}
 
 	w, waiting := g.waiting[client]
 	if !found && waiting {
@@ -234,6 +243,8 @@ func (g *GeoJS) answerOrWait(
 	g.ask(ctx)
 
 	if w == nil {
+		g.metrics.GeoJSUnanswered.Inc()
+
 		return "", nil // too many clients wait already
 	}
 
@@ -243,6 +254,8 @@ func (g *GeoJS) answerOrWait(
 	}
 
 	if w.late {
+		g.metrics.GeoJSUnanswered.Inc()
+
 		return "", nil
 	}
 
@@ -355,6 +368,8 @@ func (g *GeoJS) keep(
 	}
 
 	if err != nil {
+		g.metrics.GeoJSFailures.Inc()
+
 		g.retryDelay = min(max(retryDelayFactor*g.retryDelay, firstRetryDelay),
 			maxRetryDelay)
 		g.retryAt = now.Add(g.retryDelay)
@@ -399,6 +414,8 @@ func (g *GeoJS) request(
 
 	req.URL.RawQuery = "ip=" + strings.Join(addrs, ",")
 
+	g.metrics.GeoJSRequests.Inc()
+
 	res, err := g.httpClient.Do(req)
 	if err != nil {
 		// Do's error names the URL, and so the visitors' addresses, which

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)