Support the output format of Windows exporter

[fujimotos]

See https://github.com/fluent-plugins-nursery/fluent-plugin-windows-exporter.

Generally the current format is such a large binary msgpack object that
no existing filter plugin (e.g. filter_grep) cannot be used against it.

Windows plugin produces events in a better format, and it's preferable for Linux
plugin to support such format as well.

Current Format (msgpack binary)

2022-02-02 17:20:38.437816453 +0900 node_metrics: {"cmetrics":"metaver\u0002type
\u0001optsnsnodessnametime_secondsdesc*System time in seconds since epoch 
(1970).label_dictionarystatic_labelslabelsvaluests\u0016Th
ՄvalueA~\u0000\u0000"}

The same record in a new format

2022-02-02 17:20:38.437816453 +0900 node_metrics {"type": "gauge", "name":
"node_time_seconds", "value": 1643790130.0 }