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/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) {