Do not allow metrics with trailing slashes (#3007)

It is not possible to encode a measurement, tag, or field whose last
character is a backslash due to it being an unescapable character.
Because the tight coupling between line protocol and the internal metric
model, prevent metrics like this from being created.

Measurements with a trailing slash are not allowed and the point will be
dropped.  Tags and fields with a trailing a slash will be dropped from
the point.