add SubjectOption

Author: scotthart

google/cloud/credentials.h

@@ -458,7 +458,9 @@ struct DelegatesOption {
 };
 
 /**
- * Configure the scopes for `MakeImpersonateServiceAccountCredentials()`
+ * Configure the scopes for `MakeImpersonateServiceAccountCredentials()`.
+ * Override the scopes for `MakeServiceAccountCredentials` and
+ * `MakeServiceAccountCredentialsFromFile()`.
  *
  * @ingroup options
  * @ingroup guac
@@ -467,6 +469,17 @@ struct ScopesOption {
   using Type = std::vector<std::string>;
 };
 
+/**
+ * Overrides the subject for `MakeServiceAccountCredentials` and
+ * `MakeServiceAccountCredentialsFromFile()`.
+ *
+ * @ingroup options
+ * @ingroup guac
+ */
+struct SubjectOption {
+  using Type = std::string;
+};
+
 /**
  * Configure the access token lifetime
  *

google/cloud/internal/oauth2_service_account_credentials.cc

@@ -256,6 +256,10 @@ CreateServiceAccountCredentialsFromJsonContents(
     info->scopes = std::move(scopes);
   }
 
+  if (options.has<SubjectOption>()) {
+    info->subject = options.get<SubjectOption>();
+  }
+
   // Verify this is usable before returning it.
   auto const tp = std::chrono::system_clock::time_point{};
   auto const components = AssertionComponentsFromInfo(*info, tp);
@@ -295,6 +299,11 @@ CreateServiceAccountCredentialsFromP12FilePath(
     std::set<std::string> scopes{s.begin(), s.end()};
     info->scopes = std::move(scopes);
   }
+
+  if (options.has<SubjectOption>()) {
+    info->subject = options.get<SubjectOption>();
+  }
+
   return StatusOr<std::shared_ptr<Credentials>>(
       std::make_shared<ServiceAccountCredentials>(*info, options,
                                                   std::move(client_factory)));