metad is a metadata server support the following features:
self semantic support. metad keep a mapping of IP and metadata, client direct request "/self", will get the metadata of current node. mapping settings is store to backend.
metadata backend support etcd (TODO support more).
support metadata local cache, so it can be used as a proxy to reducing the request pressure of backend (etcd).
api out format support json/yaml/text,and is metadata/developer friendly data structure.