Update credhub exporter

[psycofdj]

changelog

This PR includes:

• package credhub_exporter
  • bumps to v0.2.0
  • rework the metrics update strategy: they are now updated in a background goroutine and completely independent of scraping requests
  • metrics are reseted on each update which fixes the use-case where credentials are deleted from credhub
  • update of internal golang dependencies
• job credhub_exporter
  • migrates to bpm
  • adds default values to job's spec matching credhub_exporter's flags default values
• job credhub_alerts
  • get rid of max_over_time calls in alert expressions since metrics can now be scrapped every few minutes
• job credhub_dashboards
  • get rid of max_over_time calls in queries
  • add index column that was already present in metrics
• ops-files
  • manifests/operators/monitor-bosh.yml: lower credhub_exporter scrape interval and timeout from 30m and 4m to 4m and 2m respectively
  • manifests/operators/use-bpm.yml: add ops-file that adds bpm job to prometheus2 instance which is now required by credhub_exporter

about bpm

I felt more comfortable generating a bpm configuration than editing the existing credhub_exporter_ctl template which become less and less readable. However I would totally understand that introducing bpm to the release is a problem and I can rework the PR to get back to a <job>_ctl script.

blobs

If the PR is accepted, the new version of credhub_exporter must be added to the blobs

# fetch credhub exporter new release
wget https://github.com/orange-cloudfoundry/credhub_exporter/releases/download/v0.2.0/credhub_exporter_linux_amd64.zip

# add tarball to blobs
bosh add-blob credhub_exporter_linux_amd64.zip credhub_exporter/credhub_exporter-0.2.0-linux_amd64-0.2-0.zip

# remove old blob
bosh remove-blob credhub_exporter/credhub_exporter-0.1.6.linux-amd64.tar.gz

[psycofdj]

@frodenas any news on this PR ?

[frodenas]

Thanks @psycofdj!