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

plugins/inputs/win_perf_counters/win_perf_counters.go

@@ -107,7 +107,8 @@ type item struct {
 	counterHandle win.PDH_HCOUNTER
 }
 
-var sanitizedChars = strings.NewReplacer("/sec", "_persec", "/Sec", "_persec", " ", "_")
+var sanitizedChars = strings.NewReplacer("/sec", "_persec", "/Sec", "_persec",
+	" ", "_", "%", "Percent", `\`, "")
 
 func (m *Win_PerfCounters) AddItem(metrics *itemList, query string, objectName string, counter string, instance string,
 	measurement string, include_total bool) {
@@ -299,13 +300,12 @@ func (m *Win_PerfCounters) Gather(acc telegraf.Accumulator) error {
 							tags["instance"] = s
 						}
 						tags["objectname"] = metric.objectName
-						fields[sanitizedChars.Replace(string(metric.counter))] = float32(c.FmtValue.DoubleValue)
+						fields[sanitizedChars.Replace(metric.counter)] =
+							float32(c.FmtValue.DoubleValue)
 
-						var measurement string
-						if metric.measurement == "" {
+						measurement := sanitizedChars.Replace(metric.measurement)
+						if measurement == "" {
 							measurement = "win_perf_counters"
-						} else {
-							measurement = metric.measurement
 						}
 						acc.AddFields(measurement, fields, tags)
 					}