add token expiration gauge metric

* New TokenRecorder with Gatherer and Set methods
* New gauge metric: fastly_rt_token_expiration

also: go mod tidy

cmd/fastly-exporter/main.go

@@ -314,6 +314,20 @@ func main() {
 		defaultGatherers = append(defaultGatherers, dcs)
 	}
 
+	if !metricNameFilter.Blocked(prometheus.BuildFQName(namespace, deprecatedSubsystem, "token_expiration")) {
+		tokenRecorder := api.NewTokenRecorder(apiClient, token)
+		tg, err := tokenRecorder.Gatherer(namespace, deprecatedSubsystem)
+		if err != nil {
+			level.Error(apiLogger).Log("during", "create token gatherer", "err", err)
+		} else {
+			err = tokenRecorder.Set(context.Background())
+			if err != nil {
+				level.Error(apiLogger).Log("during", "set token gauge metric", "err", err)
+			}
+			defaultGatherers = append(defaultGatherers, tg)
+		}
+	}
+
 	var registry *prom.Registry
 	{
 		registry = prom.NewRegistry(programVersion, namespace, deprecatedSubsystem, metricNameFilter, defaultGatherers)

go.mod

@@ -1,6 +1,6 @@
 module github.com/fastly/fastly-exporter
 
-go 1.17
+go 1.21
 
 require (
 	github.com/cespare/xxhash v1.1.0
@@ -17,6 +17,7 @@ require (
 require (
 	github.com/beorn7/perks v1.0.1 // indirect
 	github.com/cespare/xxhash/v2 v2.2.0 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/go-logfmt/logfmt v0.5.1 // indirect
 	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
 	github.com/modern-go/reflect2 v1.0.2 // indirect