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.go

@@ -17,6 +17,7 @@ import (
 	"strconv"
 	"strings"
 	"time"
+	"unicode/utf8"
 )
 
 // Config is smallwebwaf's settings. A timeout, size or rate limit of zero
@@ -103,6 +104,12 @@ type Config struct {
 	StateDir             string
 	StateWriteDelay      time.Duration
 	StateCounterInterval time.Duration
+	// MetricsToken is the bearer token a scraper sends for the metrics
+	// (SWWAF_METRICS_TOKEN), "" while it is unset and the metrics are off.
+	// MetricsTopN is how many countries get series of their own in the
+	// metrics (SWWAF_METRICS_TOP_N).
+	MetricsToken string
+	MetricsTopN  int
 
 	// settings are the values read, as given or by default, for the
 	// log line at start.
@@ -119,6 +126,10 @@ const (
 	mebibyte = 1 << 20
 	gibibyte = 1 << 30
 	ipv4Bits = 32
+	// minTokenLength is the fewest characters a token may have.
+	minTokenLength = 32
+	// masked is what the log shows for a token that is set.
+	masked = "********"
 )
 
 var (
@@ -150,6 +161,7 @@ var (
 		"is not the length of an IPv4 netblock, from 0 to 32, such as 24")
 	errNotAbsolutePath = errors.New(
 		"is not an absolute path, such as /var/lib/smallwebwaf")
+	errShortToken = errors.New("is shorter than 32 characters")
 )
 
 // FromEnvironment reads the settings with lookupEnv, normally
@@ -188,6 +200,8 @@ func FromEnvironment(lookupEnv func(string) (string, bool)) (*Config, error) {
 		StateDir:             env.absolutePath("SWWAF_STATE_DIR", "/var/lib/smallwebwaf"),
 		StateWriteDelay:      env.durationNotOff("SWWAF_STATE_WRITE_DELAY", "10s"),
 		StateCounterInterval: env.durationNotOff("SWWAF_STATE_COUNTER_INTERVAL", "15m"),
+		MetricsToken:         env.token("SWWAF_METRICS_TOKEN"),
+		MetricsTopN:          env.numberNotOff("SWWAF_METRICS_TOP_N", "50"),
 	}
 
 	for _, country := range cfg.ExclusivelyAllowedCountries {
@@ -353,6 +367,26 @@ func (e *environment) absolutePath(name, defaultValue string) string {
 	return path
 }
 
+// token reads a setting that is a bearer token. Unset, it is "", which
+// switches off what it guards; set, it must be at least minTokenLength
+// characters. Neither the log nor an error shows its value.
+func (e *environment) token(name string) string {
+	value, set := e.lookupEnv(name)
+	if !set {
+		e.settings = append(e.settings, slog.String(name, ""))
+
+		return ""
+	}
+
+	e.settings = append(e.settings, slog.String(name, masked))
+
+	if utf8.RuneCountInString(value) < minTokenLength {
+		e.check(name, errShortToken)
+	}
+
+	return value
+}
+
 // parseDuration reads a duration in Go's syntax, such as 90s or 15m, a
 // whole number of days such as 7d, or off.
 func parseDuration(value string) (time.Duration, error) {