The Flow handler provides _in_ and _out_ metrics. A configuration parameter should be provided to allow filtering of metrics for only a specific direction.

This could be accomplished using a filter parameter, such as:

"filter": {
"only_directions": [
"in",
"out"
]
}
Or this could be accomplished using metric groups:

"metric_groups": {
"enable": [
"in",
"out"
]
}
Either way, it should (ideally) be the same as for the DNSv2 handler #576