parse_metrics_log: update to the new format
parse_metrics_log: update to the new format

The new diagnostics logs report more than just metrics, and thus, output
a bit differently than they did in simpler times. This patch updates the
parsing to be compatible with the new logs.

The changes this patch addresses:
- each metrics log line had a different number of fields
- the metrics log will only emit metrics from entities that have
  changed; as such, the processing of metrics has been changed; where we
  previously filled in NaNs for missing data, we now pull from the
  previous snapshot, or fill in 0 if one doesn't exist

I tested this manually by running against some metrics logs I collected
across a couple of workloads. The results seem to match what was
expected.

Change-Id: If11b7ecc93a3f64db3b7c1994f47308b3ec44029
Reviewed-on: http://gerrit.cloudera.org:8080/10693
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>
Tested-by: Kudu Jenkins

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