CASSANDRA-19015 Nodetool 'tablestats' formatting uses inconsistent significant digits

[zaaath]

Before

Nodetool reports milliseconds (ms) with anywhere from 3 to 15 significant digits. Ratios use five or sixteen decimal places. Averages use 1 or 13 decimal places.
Example:

Keyspace: system
        Read Count: 18
        Read Latency: 5.364611111111111 ms
        Write Count: 22
        Write Latency: 0.6970454545454545 ms
        Pending Flushes: 0
                Table: IndexInfo
...
                SSTable Compression Ratio: 1.14583
...
                Local read count: 1
                Local read latency: 17.084 ms
                Local write count: 1
                Local write latency: 1.916 ms
                Local read/write ratio: 1.00000
...
                Bloom filter false ratio: 0.00000
...
                Average live cells per slice (last five minutes): 1.0
                Maximum live cells per slice (last five minutes): 1
                Average tombstones per slice (last five minutes): 1.0
                Maximum tombstones per slice (last five minutes): 1
                Droppable tombstone ratio: 0.00000

After

Milliseconds use 3 decimal places
Ratios use 3 decimal places (tenths of a percent)
Averages use 2 decimal places
Example:

Keyspace: system
        Read Count: 18
        Read Latency: 5.084 ms
        Write Count: 22
        Write Latency: 0.566 ms
        Pending Flushes: 0
                Table: IndexInfo
...
                SSTable Compression Ratio: 1.146
...
                Local read count: 1
                Local read latency: 14.237 ms
                Local write count: 1
                Local write latency: 0.770 ms
                Local read/write ratio: 1.000
...
                Bloom filter false ratio: 0.000
...
                Average live cells per slice (last five minutes): 61.43
                Maximum live cells per slice (last five minutes): 72
                Average tombstones per slice (last five minutes): 1.00
                Maximum tombstones per slice (last five minutes): 1
                Droppable tombstone ratio: 0.000

How to reproduce

nodetool tablestats

Tests

ant testsome -Dtest.name=org.apache.cassandra.utils.FBUtilitiesTest
ant testsome -Dtest.name=org.apache.cassandra.tools.nodetool.stats.TableStatsPrinterTest
ant testsome -Dtest.name=org.apache.cassandra.tools.nodetool.TableStatsTest

patch by @zaaath; reviewed by @smiklosovic for CASSANDRA-19015