Add cqlsh SHOW REPLICAS by token command

It is often useful to know when a query fails or a query is slow which nodes the query was routed  too. For clusters with vnodes, determining this is not simple and requires an admin tool like 'nodetool getendpoints;. 

But, the Cassandra driver has cluster metadata which identifies which nodes are the replicas for a given token range.  It should be straightforward to support this in CQLSH.

Example with python driver:

from cassandra.cluster import Cluster

from cassandra.auth import PlainTextAuthProvider

import cassandra.metadata

from cassandra.metadata import Murmur3Token

 

auth_provider = PlainTextAuthProvider(username='cassandra', password='cassandra')

cluster = Cluster(['127.0.0.1'], auth_provider=auth_provider)

session = cluster.connect('tests')

cluster.metadata.token_map.get_replicas('tests',Murmur3Token(100))

>>> cluster.metadata.token_map.get_replicas('tests',Murmur3Token(100))

[<Host: 127.0.0.1:9042 datacenter1>]

 

Users shouldn't require access to an admin tool like nodetool to access the cluster metadata.  This is especially true for Cassandra-as-a-service offerings like AWS keyspaces or Instaclustr's managed service.