Add metric for pipelines indicating if they are up

[excalq]

Resolves #93 by adding logstash_stats_pipeline_up{pipeline_id}, with a boolean gauge metric.

Uses sort --version-sort in scripts/add_metrics_to_readme.sh for correct mixed string/number sorting of README metrics.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (f7c7c13) 100.00% compared to head (9fb9120) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #94   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           12        12           
  Lines          480       496   +16     
=========================================
+ Hits           480       496   +16     

Impacted Files	Coverage Δ
collectors/nodestats/pipeline_subcollector.go	100.00% <100.00%> (ø)

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[excalq]

Other than the test coverage issue on the impossible-to-reach return statement, this should be ready to merge. It's rebased from master.

[kuskoman]

@excalq could you rebase this PR and update metric names after that (if required)?
Thanks in advance!

[kuskoman]

@excalq after merging this PR I will release next version as a tag

[excalq]

@kuskoman Cool, I see that #101 was merged. This changes NewDescWithHelpAndLabels(), hence the conflicts here. I'll rebase now.

[kuskoman]

@excalq I allowed myself to do rebase, force-push and a little refactor, could you take a last look on the code before I merge it?

[excalq]

The show stopper is the duplicated last_timestamps metrics. I can fix that now.

[kuskoman]

@excalq can I merge it after CI is passed? I will squash the commits therefore I changed the title to be a commit message

[excalq]

The duplication is fixed (might be a good future test, btw), so everything's ready to merge. None of my comments are blockers btw, so please resolve them.