if fastly_rt_datacenter_info is blocked, do not enable datacentercache

The datacentercache is only used for building the
`fastly_rt_datacenter_info` so if that mertic is in the blocklist we can
safely disable the cache and the gatherer

cmd/fastly-exporter/main.go

@@ -269,7 +269,8 @@ func main() {
 
 	var datacenterCache *api.DatacenterCache
 	{
-		datacenterCache = api.NewDatacenterCache(apiClient, token)
+		enabled := !metricNameFilter.Blocked(prometheus.BuildFQName(namespace, deprecatedSubsystem, "datacenter_info"))
+		datacenterCache = api.NewDatacenterCache(apiClient, token, enabled)
 	}
 
 	var productCache *api.ProductCache
@@ -285,12 +286,14 @@ func main() {
 			}
 			return nil
 		})
-		g.Go(func() error {
-			if err := datacenterCache.Refresh(context.Background()); err != nil {
-				level.Warn(logger).Log("during", "initial fetch of datacenters", "err", err, "msg", "datacenter labels unavailable, will retry")
-			}
-			return nil
-		})
+		if datacenterCache.Enabled() {
+			g.Go(func() error {
+				if err := datacenterCache.Refresh(context.Background()); err != nil {
+					level.Warn(logger).Log("during", "initial fetch of datacenters", "err", err, "msg", "datacenter labels unavailable, will retry")
+				}
+				return nil
+			})
+		}
 		g.Go(func() error {
 			if err := productCache.Refresh(context.Background()); err != nil {
 				level.Warn(logger).Log("during", "initial fetch of products", "err", err, "msg", "products API unavailable, will retry")
@@ -302,7 +305,7 @@ func main() {
 	}
 
 	var defaultGatherers prometheus.Gatherers
-	{
+	if datacenterCache.Enabled() {
 		dcs, err := datacenterCache.Gatherer(namespace, deprecatedSubsystem)
 		if err != nil {
 			level.Error(apiLogger).Log("during", "create datacenter gatherer", "err", err)
@@ -331,7 +334,9 @@ func main() {
 	}
 
 	var g run.Group
-	{
+	// only setup the ticker if the datacenterCache is enabled.
+	if datacenterCache.Enabled() {
+
 		// Every datacenterRefresh, ask the api.DatacenterCache to refresh
 		// metadata from the api.fastly.com/datacenters endpoint.
 		var (

pkg/api/datacenter_cache.go

@@ -29,24 +29,29 @@ type Coördinates struct {
 // DatacenterCache polls api.fastly.com/datacenters and maintains a local cache
 // of the returned metadata. That information is exposed as Prometheus metrics.
 type DatacenterCache struct {
-	client HTTPClient
-	token  string
+	client  HTTPClient
+	token   string
+	enabled bool
 
 	mtx sync.Mutex
 	dcs []Datacenter
 }
 
 // NewDatacenterCache returns an empty cache of datacenter metadata. Use the
 // Refresh method to update the cache.
-func NewDatacenterCache(client HTTPClient, token string) *DatacenterCache {
+func NewDatacenterCache(client HTTPClient, token string, enabled bool) *DatacenterCache {
 	return &DatacenterCache{
-		client: client,
-		token:  token,
+		client:  client,
+		token:   token,
+		enabled: enabled,
 	}
 }
 
 // Refresh the cache with metadata retreived from the Fastly API.
 func (c *DatacenterCache) Refresh(ctx context.Context) error {
+	if !c.enabled {
+		return nil
+	}
 	req, err := http.NewRequestWithContext(ctx, "GET", "https://api.fastly.com/datacenters", nil)
 	if err != nil {
 		return fmt.Errorf("error constructing API datacenters request: %w", err)
@@ -109,6 +114,11 @@ func (c *DatacenterCache) Gatherer(namespace, subsystem string) (prometheus.Gath
 	return registry, nil
 }
 
+// Enabled returns true if the DatacenterCache is enabled
+func (c *DatacenterCache) Enabled() bool {
+	return c.enabled
+}
+
 type datacenterCollector struct {
 	desc  *prometheus.Desc
 	cache *DatacenterCache

pkg/api/datacenter_cache_test.go

@@ -38,7 +38,7 @@ func TestDatacenterCache(t *testing.T) {
 			var (
 				ctx    = context.Background()
 				client = testcase.client
-				cache  = api.NewDatacenterCache(client, "irrelevant token")
+				cache  = api.NewDatacenterCache(client, "irrelevant token", true)
 			)
 
 			if want, have := testcase.wantErr, cache.Refresh(ctx); !cmp.Equal(want, have) {