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
percentage characters in sampleConfig string so that fmt.Printf
will interpret them as literal percentage characters when
running 'telegraf.exe -sample-config'
closes#620