Prometheus client & win_perf_counters char changes

1. in prometheus client, do not check for invalid characters anymore,
because we are already replacing all invalid characters with regex
anyways.
2. in win_perf_counters, sanitize field name _and_ measurement name.
Also add '%' to the list of sanitized characters, because this character
is invalid for most output plugins, and can also easily cause string
formatting issues throughout the stack.
3. All '%' will now be translated to 'Percent'

closes #1430