[vtctld] migrate tablet getters

[ajm188]

Description

This migrates all tablet read-only vtctl commands, as follows:

• GetTablet $alias => GetTablet $alias
• ListShardTablets $ks/$shard => GetTablets --keyspace $ks --shard $shard (also new: GetTablets --keyspace $ks)
• ListAllTablets -cells="c1,c2,c3" => GetTablets --cells c1 --cells "c2,c3" # both work

No changes to the output format, plus support for json in the "list shard/all tablets" commands:

❯ vtctldclient --server "localhost:15999" GetTablets
zone1-0000000100 commerce 0 master SFO-M-AMASON02:15100 SFO-M-AMASON02:17100 [] 2021-01-16T19:14:34Z
zone1-0000000101 commerce 0 replica SFO-M-AMASON02:15101 SFO-M-AMASON02:17101 [] <null>
zone1-0000000102 commerce 0 rdonly SFO-M-AMASON02:15102 SFO-M-AMASON02:17102 [] <null>
❯ vtctldclient --server "localhost:15999" GetTablet zone1-000000101
{
  "alias": {
    "cell": "zone1",
    "uid": 101
  },
  "hostname": "SFO-M-AMASON02",
  "port_map": {
    "grpc": 16101,
    "vt": 15101
  },
  "keyspace": "commerce",
  "shard": "0",
  "key_range": null,
  "type": "REPLICA",
  "db_name_override": "",
  "tags": {
  },
  "mysql_hostname": "SFO-M-AMASON02",
  "mysql_port": 17101,
  "master_term_start_time": null
}
❯ vtctldclient --server "localhost:15999" GetTablets --format json
[
  {
    "alias": {
      "cell": "zone1",
      "uid": 100
    },
    "hostname": "SFO-M-AMASON02",
    "port_map": {
      "grpc": 16100,
      "vt": 15100
    },
    "keyspace": "commerce",
    "shard": "0",
    "type": 1,
    "mysql_hostname": "SFO-M-AMASON02",
    "mysql_port": 17100,
    "master_term_start_time": {
      "seconds": 1610824474,
      "nanoseconds": 426781000
    }
  },
  {
    "alias": {
      "cell": "zone1",
      "uid": 101
    },
    "hostname": "SFO-M-AMASON02",
    "port_map": {
      "grpc": 16101,
      "vt": 15101
    },
    "keyspace": "commerce",
    "shard": "0",
    "type": 2,
    "mysql_hostname": "SFO-M-AMASON02",
    "mysql_port": 17101
  },
  {
    "alias": {
      "cell": "zone1",
      "uid": 102
    },
    "hostname": "SFO-M-AMASON02",
    "port_map": {
      "grpc": 16102,
      "vt": 15102
    },
    "keyspace": "commerce",
    "shard": "0",
    "type": 3,
    "mysql_hostname": "SFO-M-AMASON02",
    "mysql_port": 17102
  }
]

Related Issue(s)

• RFC: Vtctld Service #7058
• Migrate read-only tablet/shard/keyspace commands to vtctld gRPC API #7135

Checklist

• Should this PR be backported? No
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving
• VReplication
• Cluster Management
• Build
• VTAdmin

[doeg]

Two non-blocking questions. This looks great!

[doeg]

We can't/don't test the CLI output in commands.go, right? Not a leading question, just curious!

[doeg]

Are these left as TODOs on purpose?

[doeg]

missed opportunity to call these awkWords tbh