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/state/state.go

@@ -21,6 +21,7 @@ import (
 
 	"sneak.berlin/go/smallwebwaf/internal/bans"
 	"sneak.berlin/go/smallwebwaf/internal/lookup"
+	"sneak.berlin/go/smallwebwaf/internal/metrics"
 	"sneak.berlin/go/smallwebwaf/internal/ratelimit"
 )
 
@@ -62,6 +63,8 @@ type Params struct {
 	Now func() time.Time
 	// ProcessLog receives what was read, and the writes that fail.
 	ProcessLog *slog.Logger
+	// Metrics count each file's writes.
+	Metrics *metrics.Metrics
 }
 
 // Files are the state files of a running smallwebwaf.
@@ -204,7 +207,7 @@ func (f *Files) writeBans() error {
 		return fmt.Errorf("encode %s: %w", bansJSON, err)
 	}
 
-	return write(f.params.Dir, bansJSON, append(data, '\n'))
+	return f.writeCounted(bansJSON, append(data, '\n'))
 }
 
 // writeClients writes clients.json.
@@ -214,7 +217,7 @@ func (f *Files) writeClients() error {
 		return fmt.Errorf("encode %s: %w", clientsJSON, err)
 	}
 
-	return write(f.params.Dir, clientsJSON, data)
+	return f.writeCounted(clientsJSON, data)
 }
 
 // writeLookups writes lookups.json.
@@ -224,7 +227,16 @@ func (f *Files) writeLookups() error {
 		return fmt.Errorf("encode %s: %w", lookupsJSON, err)
 	}
 
-	return write(f.params.Dir, lookupsJSON, data)
+	return f.writeCounted(lookupsJSON, data)
+}
+
+// writeCounted writes data to the state file name, as write does, and
+// counts the write in the metrics.
+func (f *Files) writeCounted(name string, data []byte) error {
+	err := write(f.params.Dir, name, data)
+	f.params.Metrics.StateFileWritten(name, len(data), err)
+
+	return err
 }
 
 // newBanEntry returns ban as bans.json holds it.