common: Make data size and speed formatters static

common: Make data size and speed formatters static

They cannot be constructed anymore, they can be retrieved through a

Also fix the format when the type is not a Number, it should still append the
value to the string.

Change-Id: I42968def6be830589caac3c824d85a5056121f30
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/71172
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Hudson CI