[obs] reduce false positives for GitpodImageBuildDurationAnomaly

Author: kylos101

operations/observability/mixins/workspace/rules/central/image-builder.yaml

@@ -18,12 +18,16 @@ spec:
           severity: critical
         annotations:
           runbook_url: https://github.com/gitpod-io/runbooks/blob/main/runbooks/GitpodImagebuildDurationAnomaly.md
-          summary: image-builder duration is unusually high in cluster {{ $labels.cluster }}
-          description: Users are waiting too long for image builds
+          summary: image builds are happening too frequently in cluster {{ $labels.cluster }}
+          description: Users are waiting more often for image builds
         expr: |
-          (avg_over_time(gitpod_ws_manager_mk2_workspace_phase_total{phase="Running", type="ImageBuild", cluster!~"ephemeral.*"}[1h])-avg_over_time(gitpod_ws_manager_mk2_workspace_phase_total{phase="Running", type="ImageBuild", cluster!~"ephemeral.*"}[1d]))
-          /
-          stddev_over_time(gitpod_ws_manager_mk2_workspace_phase_total{phase="Running", type="ImageBuild", cluster!~"ephemeral.*"}[14d]) >=2.0
+          (
+              (
+                  avg_over_time(gitpod_ws_manager_mk2_workspace_phase_total{phase="Running", type="ImageBuild", cluster!~"ephemeral.*"}[4h])
+                  - avg_over_time(gitpod_ws_manager_mk2_workspace_phase_total{phase="Running", type="ImageBuild", cluster!~"ephemeral.*"}[7d])
+              )
+              / stddev_over_time(gitpod_ws_manager_mk2_workspace_phase_total{phase="Running", type="ImageBuild", cluster!~"ephemeral.*"}[30d])
+          ) > 2.5
       - alert: GitpodImageBuilderCrashlooping
         labels:
           severity: critical