Implement option to output information as OpenMetrics time series

[lelutin]

This new -o option will make needrestart output information in a format that can be scraped by Prometheus or any other daemon that ingests OpenMetrics format.

The -l, -w and -k options can be used in combination with -o in order to choose what information gets exported.

(Closes: #291)

[lelutin]

note, I've chosen to split the version information out into a dedicated ...build_info time series. This makes it possible to graph the timestamp normally without risking discontinuity if needrestart and/or perl is upgraded and thus the labels change, and the dedicated metric can be used just for aggregation if needs be.

[lelutin]

I've reworked this branch to incorporate feedback from @anarcat and other details found while reading the OpenMetrics spec more in depth and I've just force-pushed the result.

what's to be expected as being changed in this force-push;

• the -l option in combination with -o now requires root access, otherwise the metric for outdated libraries was always set to 0 since an unprivileged user can't dig around all running processes to find out this information
• the code and metrics now talk about "outdated libraries" and "processes with outdated libraries", which should be more precise for what we're exposing
• the metrics were renamed to be more descriptive of what they expose
• the man page now has some description of which metrics will be exposed and their types
• the timestamp metric was removed
• the info-type metrics for kernel and microcode, which were wrongly typed and implemented, now use the StateSet type to expose all of the possible states
• there's an additional info-type metric for kernel and microcode in order to expose the current and expected versions. This is instead of attaching those versions as labels to all entries in the StateSet metrics. it reduces useless duplication and still makes it possible to group by those metrics if needed in order to filter or add those labels to results
• the metadata for all MetricFamilies were reordered as per the OpenMetric recommendation (type, then help -- unit was not used)
• a trailing # EOF\n was added to properly end the exposition as defined by OpenMetrics

[liske]

Thanks!

[anarcat]

awesome, thanks for the fixes