Take in an admin's edits of the state files while running (closes #68)

smallwebwaf watches SWWAF_STATE_DIR with fsnotify and takes in a saved
edit of a state file in place of what it held. It tells its own writes
from an admin's by the SHA-256 of what it last read or wrote; each write
first takes in an edit made since. An edit that does not parse is
renamed to <name>.bad at the file's next write. Every ban on a netblock
is checked, and the next ban is worked out from the one that ended
last. Two metrics count the edits taken in and set aside. README.md
says how to add and lift a ban.

Judgement call: a broken edit is set aside at the next write, since an
editor's file can be read half written.

Model: opus-5-5

internal/metrics/metrics.go

@@ -44,6 +44,8 @@ type Metrics struct {
 	stateFileWriteFailures *prometheus.CounterVec
 	stateFileLastWrite     *prometheus.GaugeVec
 	stateFileSize          *prometheus.GaugeVec
+	stateFileEditsTakenIn  *prometheus.CounterVec
+	stateFileEditsSetAside *prometheus.CounterVec
 }
 
 // New returns the metrics, with the Go runtime's and the process's own.
@@ -105,6 +107,11 @@ func New(topN int) *Metrics {
 			"When each state file was last written, in seconds since 1970.", byFile),
 		stateFileSize: gaugeVec("smallwebwaf_state_file_size_bytes",
 			"The size of each state file, as it was last written.", byFile),
+		stateFileEditsTakenIn: counterVec("smallwebwaf_state_file_edits_taken_in_total",
+			"Edits of each state file taken in while running.", byFile),
+		stateFileEditsSetAside: counterVec("smallwebwaf_state_file_edits_set_aside_total",
+			"Edits of each state file renamed to <name>.bad because they did not parse.",
+			byFile),
 	}
 
 	m.handler = promhttp.HandlerFor(m.registry, promhttp.HandlerOpts{})
@@ -120,6 +127,7 @@ func New(topN int) *Metrics {
 		m.GeoJSRequests, m.GeoJSFailures, m.GeoJSUnanswered,
 		m.stateFileWrites, m.stateFileWriteFailures,
 		m.stateFileLastWrite, m.stateFileSize,
+		m.stateFileEditsTakenIn, m.stateFileEditsSetAside,
 	)
 
 	return m
@@ -230,6 +238,18 @@ func (m *Metrics) StateFileWritten(name string, size int, err error) {
 	m.stateFileSize.WithLabelValues(name).Set(float64(size))
 }
 
+// StateFileEditTakenIn counts an admin's edit of the state file name
+// taken in while smallwebwaf runs.
+func (m *Metrics) StateFileEditTakenIn(name string) {
+	m.stateFileEditsTakenIn.WithLabelValues(name).Inc()
+}
+
+// StateFileEditSetAside counts an admin's edit of the state file name
+// renamed to name.bad because it did not parse.
+func (m *Metrics) StateFileEditSetAside(name string) {
+	m.stateFileEditsSetAside.WithLabelValues(name).Inc()
+}
+
 // statusClass returns the class of status, such as 2xx, or none when no
 // status was sent.
 func statusClass(status int) string {