feat(repometadata): handle APIs with no service config (#4230)

Removes a check to see whether there's a service config. We never access
the service config directly within repometadata.FromAPI, so it's fine
for it to be absent. We may well have very little information to put in
the RepoMetadata returned by functions, but that's okay.

Author: jskeet

internal/repometadata/repometadata.go

@@ -36,8 +36,7 @@ const (
 )
 
 var (
-	errNoAPIs          = errors.New("library has no APIs from which to get metadata")
-	errNoServiceConfig = errors.New("library has no service config from which to get metadata")
+	errNoAPIs = errors.New("library has no APIs from which to get metadata")
 )
 
 // RepoMetadata represents the .repo-metadata.json file structure.
@@ -122,9 +121,6 @@ func FromLibrary(config *config.Config, library *config.Library, googleapisDir s
 	if err != nil {
 		return nil, fmt.Errorf("failed to find API for path %s: %w", firstAPIPath, err)
 	}
-	if api.ServiceConfig == "" {
-		return nil, fmt.Errorf("failed to generate metadata for %s: %w", library.Name, errNoServiceConfig)
-	}
 	return fromAPI(config, api, library), nil
 }
 

internal/repometadata/repometadata_test.go

@@ -74,6 +74,20 @@ func TestFromLibrary(t *testing.T) {
 				APIDescription:       "Stores, manages, and secures access to application secrets.",
 			},
 		},
+		{
+			name: "no service config",
+			library: &config.Library{
+				Name:         "google-longrunning",
+				ReleaseLevel: "stable",
+				APIs:         []*config.API{{Path: "google/longrunning"}},
+			},
+			want: &RepoMetadata{
+				ReleaseLevel:     "stable",
+				Language:         "python",
+				Repo:             "googleapis/google-cloud-python",
+				DistributionName: "google-longrunning",
+			},
+		},
 	} {
 		t.Run(test.name, func(t *testing.T) {
 			tmpDir := t.TempDir()
@@ -122,15 +136,6 @@ func TestFromLibrary_Error(t *testing.T) {
 			},
 			// Error returned by serviceconfig.Find isn't easily distinguished
 		},
-		{
-			name: "no service config",
-			library: &config.Library{
-				Name:         "google-longrunning",
-				ReleaseLevel: "stable",
-				APIs:         []*config.API{{Path: "google/longrunning"}},
-			},
-			wantErr: errNoServiceConfig,
-		},
 	} {
 		t.Run(test.name, func(t *testing.T) {
 			cfg := &config.Config{Language: serviceconfig.LangPython}