internal/gapicgen: add billing to microgen config

Test generation: https://code-review.googlesource.com/c/gocloud/+/51871

Change-Id: I42375dc2dd7cebc75d86630686b6c4ae6c9428e1
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/51872
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Noah Dietz <ndietz@google.com>
diff --git a/internal/gapicgen/generator/config.go b/internal/gapicgen/generator/config.go
index 6cefa56..005836f 100644
--- a/internal/gapicgen/generator/config.go
+++ b/internal/gapicgen/generator/config.go
@@ -57,6 +57,14 @@
 		releaseLevel:          "alpha",
 	},
 	{
+		inputDirectoryPath:    "google/cloud/billing/v1",
+		pkg:                   "billing",
+		importPath:            "cloud.google.com/go/billing/apiv1",
+		gRPCServiceConfigPath: "google/cloud/billing/v1/cloud_billing_grpc_service_config.json",
+		apiServiceConfigPath:  "google/cloud/billing/v1/cloudbilling.yaml",
+		releaseLevel:          "beta",
+	},
+	{
 		inputDirectoryPath:    "google/cloud/language/v1",
 		pkg:                   "language",
 		importPath:            "cloud.google.com/go/language/apiv1",