[MetricBeat] Add up metric for health status

[premendrasingh]

Enhancement: This PR adds up metric to metrics scraped from host. It will be useful for instance availability monitoring.
Sample metric:
up{job="prometheus",instance="localhost:8080"} 1

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[exekias]

Thank you so much for contributing 🎉 ! This PR looks good to me. Could you please check the lint errors? mage fmt would probably fix this.

[premendrasingh]

@exekias fixed lint error. Thanks for the suggestion.

[exekias]

Tests seem to be failing, we use some golden files to test the output of the module, you can update them with this command:

go test github.com/elastic/beats/metricbeat/module/prometheus/collector -data

[exekias]

sorry, wrong button 😇

[premendrasingh]

Executed
go test github.com/elastic/beats/metricbeat/module/prometheus/collector -data
Thanks for the suggestion.

[exekias]

ok to test

[premendrasingh]

Thanks a lot @exekias

[exekias]

Thank you!!