" is a valid char in metric names

[dgryski]

Figure out the full set. Figure out which subset we want to support. Hope we don't have to fully break expression parsing in the meantime.

[nnuss]

A read-through of the graphite grammar gives me: ASCII 33 thru 126 (pyparsing printables):

!"#$%&'()*+,-./
0123456789
:;<=>?@
ABCDEFGHIJKLMNOPQRSTUVWXYZ
[\]^_`
abcdefghijklmnopqrstuvwxyz
{|}~

While these symbols must be \ escaped if present as part of the metric name:

(){},=.'"\

/cc @kanatohodets

[Civil]

Do we need any extra work in this place? Or we should close this and improve handling of the cases mentioned in #175?