public Optional<String> getValue(Dimension dimension) {
        return Optional.ofNullable(map.get(dimension));
    }
Denotes which RawFlag should be retrieved from {@link FlagSource} for a given {@link FlagId},
as the raw flag may depend on the hostname, application, etc.

@author hakonhall
flags/src/main/java/com/yahoo/vespa/flags/FetchVector.java