Update handlers to match stats_prometheus.go

#1051 updated stats_prometheus.go to replace `.` for `_`, but the code that then adds the labels to every instance did not update it, so instances where CUSTOM_FIELDS is set to use a key with `.`s leads to panics.

Signed-off-by: Rubens Farias <rubens.mbfarias@gmail.com>

handlers.go

@@ -187,7 +187,7 @@ func newFalcoPayload(payload io.Reader) (types.FalcoPayload, error) {
 		}
 	}
 	for key := range config.Templatedfields {
-		if regPromLabels.MatchString(key) {
+		if regPromLabels.MatchString(strings.ReplaceAll(i, ".", "_")) {
 			promLabels[key] = fmt.Sprintf("%v", falcopayload.OutputFields[key])
 		}
 	}