Look clients up in the IPinfo Lite file with SWWAF_LOOKUP_SOURCE=file (closes #22)

SWWAF_LOOKUP_SOURCE=file looks every client up in the file
SWWAF_LOOKUP_DB_PATH names, without GeoJS. file without the path, the
path with another source, or a file that cannot be read stops the start.
The file is read whole into memory, so overwriting it in place cannot
disturb a lookup, and read again 2 seconds after its last change; a
replacement that cannot be read is logged, counted and sent as a
file_error alert, and the old one stays in use. Metrics give when it
was read and the failed reads. Tests write their databases through
internal/lookup/lookuptest.

Deviation: go.mod and go.sum written by hand; go runs only through make.
Judgement call: the 2-second wait, as the rule files have.

Model: opus-5-5

internal/metrics/metrics.go

@@ -229,6 +229,28 @@ func (m *Metrics) AddRemoteLog(remote *remotelog.Sender) {
 	)
 }
 
+// AddLookupFile adds the metrics of the lookup database, read as the
+// metrics are asked for: when the file in use was read, which lastRead
+// returns, and the replacements of it that could not be read, which
+// readFailures returns. The lookup package's File, which has both, cannot
+// be named here: that package counts GeoJS's requests in these metrics.
+func (m *Metrics) AddLookupFile(lastRead func() time.Time, readFailures func() int) {
+	m.registry.MustRegister(
+		prometheus.NewGaugeFunc(prometheus.GaugeOpts{
+			Name: "smallwebwaf_lookup_database_last_read_timestamp_seconds",
+			Help: "When the lookup database in use was read, in seconds since 1970.",
+		}, func() float64 {
+			return float64(lastRead().Unix())
+		}),
+		prometheus.NewCounterFunc(prometheus.CounterOpts{
+			Name: "smallwebwaf_lookup_database_read_failures_total",
+			Help: "Replacements of the lookup database that could not be read.",
+		}, func() float64 {
+			return float64(readFailures())
+		}),
+	)
+}
+
 // AddAlerts adds the metrics of the alerts sent to each destination set,
 // read from queue as the metrics are asked for, by destination: the
 // alerts sent, the requests to the destination that failed, the alerts