chore: add enumeration data type to .repo-metadata.json (#33)

Adds a standard enumeration of library types in .repo-metadata.json

.repo-metadata.json

@@ -1,15 +1,16 @@
 {
-  "name": "api-gateway",
-  "name_pretty": "API Gateway",
-  "product_documentation": "https://cloud.google.com/api-gateway/docs",
-  "api_description": "enables you to provide secure access to your backend services through a well-defined REST API that is consistent across all of your services, regardless of the service implementation. Clients consume your REST APIS to implement standalone apps for a mobile device or tablet, through apps running in a browser, or through any other type of app that can make a request to an HTTP endpoint.",
-  "client_documentation": "https://googleapis.dev/java/google-cloud-api-gateway/latest/index.html",
-  "release_level": "ga",
-  "transport": "grpc",
-  "language": "java",
-  "repo": "googleapis/java-api-gateway",
-  "repo_short": "java-api-gateway",
-  "distribution_name": "com.google.cloud:google-cloud-api-gateway",
-  "api_id": "api-gateway.googleapis.com",
-  "requires_billing": true
-}
+    "name": "api-gateway",
+    "name_pretty": "API Gateway",
+    "product_documentation": "https://cloud.google.com/api-gateway/docs",
+    "api_description": "enables you to provide secure access to your backend services through a well-defined REST API that is consistent across all of your services, regardless of the service implementation. Clients consume your REST APIS to implement standalone apps for a mobile device or tablet, through apps running in a browser, or through any other type of app that can make a request to an HTTP endpoint.",
+    "client_documentation": "https://googleapis.dev/java/google-cloud-api-gateway/latest/index.html",
+    "release_level": "ga",
+    "transport": "grpc",
+    "language": "java",
+    "repo": "googleapis/java-api-gateway",
+    "repo_short": "java-api-gateway",
+    "distribution_name": "com.google.cloud:google-cloud-api-gateway",
+    "api_id": "api-gateway.googleapis.com",
+    "requires_billing": true,
+    "library_type": "GAPIC_AUTO"
+}