Cannot collect Checkpoint metrics from Flink version 1.2 or later

[gom]

Hi,
I tried flink_exporter with Flink v1.4.1.
But following errors are occured.

ERRO[2018-02-26T16:47:13+09:00] js.Get 'count' = invalid value type

flink_exporter tries to get count field from /jobs/<job id>/checkpoints,
but Flink v1.4.1 exposes only counts...

In Flink version 1.2 or later, checkpoint statistics are changed.
https://issues.apache.org/jira/browse/FLINK-4410

• count is changed to counts and expose more detail (ex. in_progress, failed...)
• Add latest.completed that summary of the latest completed checkpoint.
• history are added more information.
  and so on...

At least, we should fix checkpoints count and latest history with Flink v1.2 or later.