Metricbeat - Monitor Stopped Processes

**Describe the enhancement:**
Add the ability to report if a process is not running in addition to its current process state. Possibly add an additional flag so that in additional to returning running or sleeping it will also report "not running" or "stopped" if the process is not part of the process list.

**Describe a specific use case for the enhancement or feature:**
The ability to report if a process is not running regardless of why it stopped, when it stopped and if it never started. This can be used for alerting on processes that can't use heartbeat.

Example system.yml:

- module: system
period: 10s
metricsets:
- process
- report-not-running
processes: ['kibana']

In the above scenario metricbeat would check every 10s and report the process state if it is running or sleeping, but it would also report that the process is not running if Metricbeat cannot find it in the running process list.

Start by reading the Metricbeat system module's process metricset and its configuration handling. Trace how configured processes are matched and how running or sleeping states are reported. Done means a configured process absent from the process list produces a distinct stopped or not-running state on each reporting interval, while existing states remain unchanged.

Written by the indexing model from the issue text.