build: cmake: build api/api-doc/metrics.json

metrics.json was added in d694a42745,
`configure.py` was updated accordingly. this change mirrors this
change in CMake building system.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Closes #14753

api/CMakeLists.txt

@@ -14,6 +14,7 @@ set(swagger_files
   api-doc/hinted_handoff.json
   api-doc/lsa.json
   api-doc/messaging_service.json
+  api-doc/metrics.json
   api-doc/storage_proxy.json
   api-doc/storage_service.json
   api-doc/stream_manager.json