Ensure protobuf metadata cache is predictably started when the cache manager has finished initializing

Casual code browsing led me to ProtobufMetadataCacheStartedTest where I see we actually assert that protobuf metadata cache is not started on cache manager start, it is actually started lazily, when first non-internal cache starts. And the code is written as if this is a (happy) accident. That's very dangerous. We should start it predictably, when the cache manager has finished startup.