Rename gcs to CloudStorage. (#774)

A future commit will add GCE support, and having two three-letter module names that only differ in one letter is not very developer-friendly.
diff --git a/metrics/main.go b/metrics/main.go
index 35a3104..b5e1db0 100644
--- a/metrics/main.go
+++ b/metrics/main.go
@@ -59,9 +59,9 @@
 	}
 	bk := clients.CreateCachedBuildkiteClient(bkAPI, time.Duration(settings.BuildkiteCacheTimeoutMinutes)*time.Minute)
 
-	gcs, err := clients.CreateGcsClient()
+	storageClient, err := clients.CreateCloudStorageClient()
 	if err != nil {
-		log.Fatalf("Cannot create GCS client: %v", err)
+		log.Fatalf("Cannot create Cloud Storage client: %v", err)
 	}
 
 	/*
@@ -97,7 +97,7 @@
 	criticalPath := metrics.CreateCriticalPath(bk, 20, pipelines...)
 	srv.AddMetric(criticalPath, minutes(60), defaultPublisher)
 
-	flakiness := metrics.CreateFlakiness(gcs, "bazel-buildkite-stats", "flaky-tests-bep", pipelines...)
+	flakiness := metrics.CreateFlakiness(storageClient, "bazel-buildkite-stats", "flaky-tests-bep", pipelines...)
 	srv.AddMetric(flakiness, minutes(60), defaultPublisher)
 
 	macPerformance := metrics.CreateMacPerformance(bk, 20, pipelines...)