Werk #13987: job: replace useless start time metric with job age

Component	Checks & agents
Title	job: replace useless start time metric with job age
Date	May 23, 2022
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.2.0b1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk Cloud (CCE), Checkmk MSP (CME) 2.1.0p1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 2.0.0p25 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

The check plugin job, which monitors cronjobs running with mk-job, produced a useless metric measuring the unix time stamp at which the job had been started. This metric has now been replaced with a new metric job age, which measures the actual age of the job.

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