fix(generator): include API version in client docs

Author: noahdietz

generator/integration_tests/golden/v1/golden_kitchen_sink_client.h

@@ -38,6 +38,8 @@ GOOGLE_CLOUD_CPP_INLINE_NAMESPACE_BEGIN
 ///
 /// GoldenKitchenSinkClient
 ///
+/// This client uses GoldenKitchenSink version test-api-version.
+///
 /// @par Equality
 ///
 /// Instances of this class created via copy-construction or copy-assignment

generator/internal/format_class_comments.cc

@@ -18,6 +18,7 @@
 #include "google/cloud/internal/absl_str_replace_quiet.h"
 #include "google/cloud/log.h"
 #include "absl/strings/strip.h"
+#include <google/api/client.pb.h>
 
 namespace google {
 namespace cloud {
@@ -78,6 +79,12 @@ std::string FormatClassCommentsFromServiceComments(
          {service.name(), service_name}});
   }
 
+  auto api_version = service.options().GetExtension(google::api::api_version);
+  if (!api_version.empty()) {
+    absl::StrAppend(&formatted_comments,
+                    "\n/// This client uses ", service_name, " version ", api_version, ".\n");
+  }
+
   auto const references =
       ResolveCommentReferences(formatted_comments, *service.file()->pool());
   auto trailer = std::string{};