@Override
    public Optional<RawFlag> fetch(FlagId id, FetchVector vector) {
        return Optional.ofNullable(flagData.get(id)).flatMap(data -> data.resolve(vector));
    }
A flag source that can be used in emergencies, see test for path and format of file.

<p>Use file if the ZooKeeper flag makes it difficult to start the config server, would make irreparable damage,
or is impossible to change through ZooKeeper due to a bug. This flag source should take precedence over ZooKeeper
whenever it has specified the value for a flag.

@author hakonhall
configserver-flags/src/main/java/com/yahoo/vespa/configserver/flags/db/BootstrapFlagSource.java