PIP-135: Include MetadataStore backend for Etcd

[merlimat]

Motivation

Since all the pieces that composed the proposal in PIP-45 were finally merged
and are currently ready for 2.10 release, it is now possible to add other
metadata backends that can be used to support a BookKeeper + Pulsar cluster.

One of the popular systems that is most commonly used in alternative of
ZooKeeper is Etcd, thus it makes sense to have this as the first non-zookeeper
implementation.

Goal

Provide a Etcd implementation for the MetadataStore API. This will allow
users to deploy Pulsar clusters using Etcd service for the metadata and it will
not require the presence of ZooKeeper.

Implementation

• Use the existing JEtcd Java client library for Etcd
• Extends the AbstractBatchedMetadataStore class, in order to reuse the
  transparent batching logic that will be shared with the ZookKeeper
  implementation.

Work in progress: #13225

[hpvd]

not sure:
is this somehow related/interesting: https://bookkeeper.apache.org/bps/BP-28-etcd-as-metadata-store/

edit: sorry, unqualified, since you work on bk..

[merlimat]

@hpvd We went through a different route here:

• Provide a generic KV pluggable interface for Pulsar metadata access (MetadataStore APIs)
• Implement Bookkeeper metadata access on top of Pulsar MetadataStore interfaces.

BookKeeper is now able to use any of the Pulsar MetadataStore implementations.

[hpvd]

@merlimat
many thanks for explanation!