Until I tried enabling the "Treat absent signals as 0" setting, the contents of the samples file was nearly empty.
cat factorystatsd-samples.json
{"entities":[{"settings":{"name":"","tags":"","absent_signals":"treat-as-0"},"red_signals":[{"signal":{"type":"virtual","name":"signal-T"},"count":4299},{"signal":{"type":"virtual","name":"signal-X"},"count":123327},{"signal":{"type":"virtual","name":"signal-Y"},"count":123336},{"signal":{"type":"virtual","name":"signal-Z"},"count":3879},{"signal":{"type":"item","name":"coal"},"count":3},{"signal":{"type":"item","name":"iron-ore"},"count":13},{"signal":{"type":"item","name":"copper-ore"},"count":6},{"signal":{"type":"item","name":"copper-plate"},"count":11},{"signal":{"type":"item","name":"steel-plate"},"count":2},{"signal":{"type":"item","name":"electronic-circuit"},"count":1},{"signal":{"type":"item","name":"flying-robot-frame"},"count":1},{"signal":{"type":"item","name":"rocket-control-unit"},"count":2}]},{"settings":{"name":"","tags":"","absent_signals":"treat-as-0"},"green_signals":[{"signal":{"type":"fluid","name":"crude-oil"},"count":9},{"signal":{"type":"fluid","name":"sulfuric-acid"},"count":4},{"signal":{"type":"fluid","name":"lubricant"},"count":2},{"signal":{"type":"item","name":"coal"},"count":-1},{"signal":{"type":"item","name":"stone"},"count":1},{"signal":{"type":"item","name":"iron-ore"},"count":-11},{"signal":{"type":"item","name":"copper-ore"},"count":-6},{"signal":{"type":"item","name":"uranium-ore"},"count":2},{"signal":{"type":"item","name":"iron-plate"},"count":6},{"signal":{"type":"item","name":"copper-plate"},"count":-11},{"signal":{"type":"item","name":"steel-plate"},"count":-1},{"signal":{"type":"item","name":"plastic-bar"},"count":9},{"signal":{"type":"item","name":"sulfur"},"count":6},{"signal":{"type":"item","name":"battery"},"count":3},{"signal":{"type":"item","name":"electronic-circuit"},"count":6},{"signal":{"type":"item","name":"advanced-circuit"},"count":18},{"signal":{"type":"item","name":"processing-unit"},"count":4},{"signal":{"type":"item","name":"flying-robot-frame"},"count":-1},{"signal":{"type":"item","name":"rocket-control-unit"},"count":-1},{"signal":{"type":"item","name":"low-density-structure"},"count":2},{"signal":{"type":"item","name":"rocket-fuel"},"count":7},{"signal":{"type":"item","name":"automation-science-pack"},"count":2},{"signal":{"type":"item","name":"logistic-science-pack"},"count":1},{"signal":{"type":"item","name":"military-science-pack"},"count":1},{"signal":{"type":"item","name":"chemical-science-pack"},"count":2},{"signal":{"type":"item","name":"utility-science-pack"},"count":2}]},{"settings":{"name":"","tags":"","absent_signals":"treat-as-0"},"green_signals":[{"signal":{"type":"virtual","name":"signal-T"},"count":4299},{"signal":{"type":"virtual","name":"signal-X"},"count":123327},{"signal":{"type":"virtual","name":"signal-Y"},"count":123336},{"signal":{"type":"virtual","name":"signal-Z"},"count":3879},{"signal":{"type":"fluid","name":"crude-oil"},"count":9},{"signal":{"type":"fluid","name":"sulfuric-acid"},"count":4},{"signal":{"type":"fluid","name":"lubricant"},"count":2},{"signal":{"type":"item","name":"coal"},"count":2},{"signal":{"type":"item","name":"stone"},"count":1},{"signal":{"type":"item","name":"iron-ore"},"count":2},{"signal":{"type":"item","name":"uranium-ore"},"count":2},{"signal":{"type":"item","name":"iron-plate"},"count":6},{"signal":{"type":"item","name":"steel-plate"},"count":1},{"signal":{"type":"item","name":"plastic-bar"},"count":9},{"signal":{"type":"item","name":"sulfur"},"count":6},{"signal":{"type":"item","name":"battery"},"count":3},{"signal":{"type":"item","name":"electronic-circuit"},"count":7},{"signal":{"type":"item","name":"advanced-circuit"},"count":18},{"signal":{"type":"item","name":"processing-unit"},"count":4},{"signal":{"type":"item","name":"rocket-control-unit"},"count":1},{"signal":{"type":"item","name":"low-density-structure"},"count":2},{"signal":{"type":"item","name":"rocket-fuel"},"count":7},{"signal":{"type":"item","name":"automation-science-pack"},"count":2},{"signal":{"type":"item","name":"logistic-science-pack"},"count":1},{"signal":{"type":"item","name":"military-science-pack"},"count":1},{"signal":{"type":"item","name":"chemical-science-pack"},"count":2},{"signal":{"type":"item","name":"utility-science-pack"},"count":2}]}]}%
The sample file disappears immediately after being written, which I assume means it got forwarded. I haven't seen any factorio metrics show up yet but I'm unsure if that's a separate problem.