Adding new TUnit values to runtime profile is broken

See IMPALA-8235 for context.

The problem this tracks is that there's no way to add a new value to the TUnit enum, which is required in TCounter, without breaking existing readers (at least Java readers), which will fail to decode TCounter values with an enum value they don't recognise.

The workaround is to only use the following units in anything that is serialised into a TCounter:

  UNIT,
  UNIT_PER_SECOND,
  CPU_TICKS,
  BYTES
  BYTES_PER_SECOND,
  TIME_NS,
  DOUBLE_VALUE