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/config/config_test.go

@@ -44,8 +44,13 @@ const (
 	stateDir                = "SWWAF_STATE_DIR"
 	stateWriteDelay         = "SWWAF_STATE_WRITE_DELAY"
 	stateCounterInterval    = "SWWAF_STATE_COUNTER_INTERVAL"
+	metricsToken            = "SWWAF_METRICS_TOKEN" //nolint:gosec // the setting's name
+	metricsTopN             = "SWWAF_METRICS_TOP_N"
 )
 
+// token is a token of 32 characters, the shortest allowed.
+const token = "0123456789abcdef0123456789abcdef"
+
 // off switches a timeout, a size limit or a rate limit off.
 const off = "off"
 
@@ -98,6 +103,8 @@ func TestDefaults(t *testing.T) {
 		StateDir:                    "/var/lib/smallwebwaf",
 		StateWriteDelay:             10 * time.Second,
 		StateCounterInterval:        15 * time.Minute,
+		MetricsToken:                "",
+		MetricsTopN:                 50,
 	})
 
 	if cfg.UpstreamURL.String() != "http://127.0.0.1:8081" {
@@ -145,6 +152,8 @@ func TestValuesAsSet(t *testing.T) {
 		stateDir:                "/srv/waf-state",
 		stateWriteDelay:         "500ms",
 		stateCounterInterval:    "1h",
+		metricsToken:            token,
+		metricsTopN:             "10",
 	})
 
 	wantSettings(t, cfg, config.Config{
@@ -169,6 +178,8 @@ func TestValuesAsSet(t *testing.T) {
 		StateDir:                    "/srv/waf-state",
 		StateWriteDelay:             500 * time.Millisecond,
 		StateCounterInterval:        time.Hour,
+		MetricsToken:                token,
+		MetricsTopN:                 10,
 	})
 
 	if cfg.UpstreamURL.String() != "https://app.internal:8443/" {
@@ -344,6 +355,7 @@ func TestInvalidValueStopsTheStart(t *testing.T) {
 		{stateDir, ""}, {stateDir, "state"}, {stateDir, "./var/lib/smallwebwaf"},
 		{stateWriteDelay, off}, {stateWriteDelay, "0s"},
 		{stateCounterInterval, off}, {stateCounterInterval, "15"},
+		{metricsTopN, off}, {metricsTopN, "0"}, {metricsTopN, "-1"},
 	} {
 		t.Run(tc.name+"="+tc.value, func(t *testing.T) {
 			t.Parallel()
@@ -360,6 +372,39 @@ func TestInvalidValueStopsTheStart(t *testing.T) {
 	}
 }
 
+func TestShortTokenStopsTheStartWithoutShowingIt(t *testing.T) {
+	t.Parallel()
+
+	// Characters are counted, not bytes: each é takes two.
+	for _, value := range []string{"", token[1:], strings.Repeat("é", 31)} {
+		t.Run(value, func(t *testing.T) {
+			t.Parallel()
+
+			_, err := config.FromEnvironment(environment{metricsToken: value}.lookupEnv)
+
+			want := metricsToken + ": is shorter than 32 characters"
+			if err == nil || err.Error() != want {
+				t.Errorf("error %v, want %s", err, want)
+			}
+		})
+	}
+}
+
+func TestTokenIsLoggedMasked(t *testing.T) {
+	t.Parallel()
+
+	cfg := fromEnvironment(t, environment{metricsToken: token})
+
+	var out bytes.Buffer
+
+	slog.New(slog.NewJSONHandler(&out, nil)).Info("starting", "settings", cfg)
+
+	if strings.Contains(out.String(), token) ||
+		!strings.Contains(out.String(), `"`+metricsToken+`":"********"`) {
+		t.Errorf("the token is not logged masked: %s", out.String())
+	}
+}
+
 func TestLogsEachSettingWithItsValue(t *testing.T) {
 	t.Parallel()
 
@@ -407,6 +452,8 @@ func TestLogsEachSettingWithItsValue(t *testing.T) {
 		stateDir:                "/var/lib/smallwebwaf",
 		stateWriteDelay:         "10s",
 		stateCounterInterval:    "15m",
+		metricsToken:            "",
+		metricsTopN:             "50",
 	}
 	if !maps.Equal(line.Settings, want) {
 		t.Errorf("logged settings\n%v\nwant\n%v", line.Settings, want)
@@ -435,7 +482,8 @@ func wantSettings(t *testing.T, got *config.Config, want config.Config) {
 	wantBanSettings(t, got, want)
 }
 
-// wantBanSettings checks the settings for bans and the state files.
+// wantBanSettings checks the settings for bans, the state files and the
+// metrics.
 func wantBanSettings(t *testing.T, got *config.Config, want config.Config) {
 	t.Helper()
 
@@ -453,6 +501,11 @@ func wantBanSettings(t *testing.T, got *config.Config, want config.Config) {
 		got.StateCounterInterval != want.StateCounterInterval {
 		t.Errorf("state settings\n%+v\nwant\n%+v", got, want)
 	}
+
+	if got.MetricsToken != want.MetricsToken || got.MetricsTopN != want.MetricsTopN {
+		t.Errorf("metrics token %q and top %d, want %q and %d",
+			got.MetricsToken, got.MetricsTopN, want.MetricsToken, want.MetricsTopN)
+	}
 }
 
 // wantNetblocks checks a list of netblocks.