JSON output logging is too noisy

[na--]

When k6 is ran in verbose mode and outputting metrics to a JSON file, the log is full of useless debug messages:

time="2020-05-28T13:35:10+03:00" level=debug msg="JSON: Writing JSON metrics" filename=output.json
time="2020-05-28T13:35:10+03:00" level=debug msg="JSON: Wrote JSON metrics" count=0 filename=output.json t="2.379µs"
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Writing JSON metrics" filename=output.json
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Wrote JSON metrics" count=0 filename=output.json t="2.391µs"
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Writing JSON metrics" filename=output.json
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Wrote JSON metrics" count=0 filename=output.json t="3.86µs"
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Writing JSON metrics" filename=output.json
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Wrote JSON metrics" count=0 filename=output.json t="4.195µs"
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Writing JSON metrics" filename=output.json
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Wrote JSON metrics" count=0 filename=output.json t="7.062µs"
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Writing JSON metrics" filename=output.json
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Wrote JSON metrics" count=0 filename=output.json t="32.689µs"
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Writing JSON metrics" filename=output.json
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Wrote JSON metrics" count=0 filename=output.json t="6.224µs"
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Writing JSON metrics" filename=output.json
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Wrote JSON metrics" count=2 filename=output.json t="103.97µs"
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Writing JSON metrics" filename=output.json
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Wrote JSON metrics" count=0 filename=output.json t="6.374µs"
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Writing JSON metrics" filename=output.json
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Wrote JSON metrics" count=0 filename=output.json t="3.807µs"
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Writing JSON metrics" filename=output.json
time="2020-05-28T13:35:11+03:00" level=debug msg="JSON: Wrote JSON metrics" count=0 filename=output.json t="4.033µs"
time="2020-05-28T13:35:12+03:00" level=debug msg="JSON: Writing JSON metrics" filename=output.json
time="2020-05-28T13:35:12+03:00" level=debug msg="JSON: Wrote JSON metrics" count=0 filename=output.json t="3.672µs"

The Writing JSON metrics message seems completely superfluous, and the Wrote JSON metrics should probably only be shown when we actually wrote something - most of these have count=0... Also, 100ms seems way to small of an interval to write metrics in a file, so investigating if bigger batches aren't better is probably worthwhile.

[imiric]

Closed by #1470.