This change is a companion to the fix for monotonic counter implementations in
spectatord.

Netflix-Skunkworks/spectatord#90

The original monotonic counter (`C`) was always intended to be used for the
case where a monotonic data source needs to be transformed into base units
for recording data. For example, transforming nanoseconds into seconds. This
requires division, which results in floats.

There is a valid use case for handling uints in monotonic counters, if the
data source is already in a base unit, such as bytes. Thus, a new meter type
`U` is added to spectatord which supports this use case.

It was using the existing `send` method, which formatted the value as a float,
when it needs to be formatted as a uint.

It is easy enough to break the spectatord line protocol, if any of the control
characters (`:,=`) are inserted in unexpected places. Since metric names and
tags are often programmatically generated, we want to only construct id strings
which are valid.

This file is generated by tools/gen_valid_chars.