Sanitize 'win_perf_counters' Counter names for Graphite

[G-regL]

When sending Telegraf data to Graphite, many of the metrics generated by the win_perf_counters input plugin aren't properly sanitized, and get ignored by Carbon.

For example, for the following config, only Processes and Threads succeed:

[[outputs.graphite]]
  template = "host.measurement.tags.field"

[[inputs.win_perf_counters]]
  [[inputs.win_perf_counters.object]]
    ObjectName = "System"
    Counters = ["Context Switches/sec", "System Calls/sec", "Threads", "System Up Time", "Processes", "Processor Queue Length"]
    Instances = ["------"]
    Measurement = "win_system"

If run in debug mode, telegraf produces this output:

> win_system,host=ServerA,objectname=System Context\ Switches/sec=9189.787 1461855980003819600
> win_system,host=ServerA,objectname=System System\ Calls/sec=41990.85 1461855980003819600
> win_system,host=ServerA,objectname=System Threads=2260 1461855980013820100
> win_system,host=ServerA,objectname=System System\ Up\ Time=1480454.9 1461855980013820100
> win_system,host=ServerA,objectname=System Processes=158 1461855980013820100
> win_system,host=ServerA,objectname=System Processor\ Queue\ Length=0 1461855980023820600

The metrics are received by carbon, but it throws these 4 errors:

28/04/2016 11:06:20 :: invalid line (ServerA.win_system.System.Context Switches/sec 9189.787 1461855980) received from client 127.0.0.1:57807, ignoring
28/04/2016 11:06:20 :: invalid line (ServerA.win_system.System.System Calls/sec 41990.85 1461855980) received from client 127.0.0.1:57807, ignoring
28/04/2016 11:06:20 :: invalid line (ServerA.win_system.System.System Up Time 1.4804549e+06 1461855980) received from client 127.0.0.1:57807, ignoring
28/04/2016 11:06:20 :: invalid line (ServerA.win_system.System.Processor Queue Length 0 1461855980) received from client 127.0.0.1:57807, ignoring

I see that the Graphite output serializer sanitizes the bucket name, but it doesn't sanitize the field itself, and win_perf_counters doesn't do any kind of sanitizing when gathering metrics.