[obs] reduce false positives for GitpodImageBuildDurationAnomaly

[kylos101]

Description

This would have caught incidents from July 13 and July 17, and avoided recent false positives on gitpod.io.

Avoiding including this with Dedicated for now, it would have triggered for dogfood yesterday.

Summary generated by Copilot

🤖 Generated by Copilot at a7be151

Renamed and modified an alert for image builder performance in operations/observability/mixins/workspace/rules/central/image-builder.yaml. The new alert detects anomalies in the frequency of image builds, rather than the duration, to better reflect the demand and health of the image builder service.

Related Issue(s)

xref: https://gitpod.slack.com/archives/C01TNS8EVQT/p1691631564908519

How to test

Documentation

Preview status

gitpod:summary

Build Options

Build

• /werft with-werft
  Run the build with werft instead of GHA
• leeway-no-cache
• /werft no-test
  Run Leeway with --dont-test

Publish

• /werft publish-to-npm
• /werft publish-to-jb-marketplace

Installer

• analytics=segment
• with-dedicated-emulation
• workspace-feature-flags
  Add desired feature flags to the end of the line above, space separated

Preview Environment / Integration Tests

• /werft with-local-preview
  If enabled this will build install/preview
• /werft with-preview
• /werft with-large-vm
• /werft with-gce-vm
  If enabled this will create the environment on GCE infra
• with-integration-tests=all
  Valid options are all, workspace, webapp, ide, jetbrains, vscode, ssh. If enabled, with-preview and with-large-vm will be enabled.
• with-monitoring

/hold