Add option to opt out of default Spanner resource filter for custom metrics

[kj455]

Background

Currently, the Poller always prepends every metric query with the following base filter (implementation):

resource.type="spanner_instance" AND  
resource.labels.instance_id="<INSTANCE_ID>" AND  
project="<PROJECT_ID>" AND …  

However, custom metrics can’t be written under the spanner_instance resource type (docs). As a result, any predictive business- or application-level KPIs that signal upcoming high load emitted under other resource types cannot be picked up by the autoscaler.

Use Case: Predictive Pre-Scaling

• Future-Load Indicator
  • A separate service publishes a Cloud Monitoring gauge metric that implies future Spanner load(e.g. estimated QPS or queue length).
• Pre-warm Nodes
  • When that predictive metric crosses a given threshold, we want to proactively add nodes before the actual load hits Spanner.
• Stable Scaling Decision
  • Because this external metric won’t drop simply by adding Spanner nodes, we need to calculate desired node count based on both the metric value and the current node count.

Proposal

1. Introduce an externalMetric flag

   • Type: boolean
   • Purpose: When set to true, disable the built-in Spanner resource filter so that any Cloud Monitoring query can be used.
   • Default: false (existing behavior)

2. Support a per-node threshold (thresholdByNode)

   • Type: number

   • Purpose: Compute the desired node count as

     desiredNodes = ceil(metricValue / thresholdByNode)
     

     This prevents unbounded scale-outs for external metrics that is not not affected by spanner nodes.

   • Default: undefined

Benefits

• Proactive scaling based on true business-level KPIs, not just realtime CPU or storage.
• Backward compatibility: existing users see no change unless they opt in.

Thank you for considering this enhancement! We’d be happy to help prototype, test, or refine the design.

[kj455]

I’ve revised the proposal and would appreciate your feedback on its adoption feasibility whenever you have a moment 🙏

[kj455]

Hi @henrybell

I hope you’re doing well. I’ve updated the proposal and would be very grateful if you could share your thoughts on its feasibility when you have a moment. Thank you for your time and consideration!