[receiver/systemd] Add metric for service restarts

[SquidDev]

Description

This adds a metric for the number of times a systemd service has exited and been automatically restarted. This is not enabled by default — I think there's enough overlap with other metrics I'd like to expose (e.g. service uptime), that it's not worth it.

Testing

There is a unit for this, and I did some manual testing with a transient unit started with systemd-run --user -u test -p Restart=always -p StartLimitBurst=50 -- bash -c "sleep 1; exit 1".

Documentation

Description (and some extended docs) in the metadata.yaml as expected, but nothing else.

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[SquidDev]

Thank you! ❤️