make ExternalMetricsRequestWindow configurable

[tav-pw]

Add configurable --external-metrics-request-window flag

Summary

Add a --external-metrics-request-window flag to make the Cloud Monitoring query window configurable. Currently hardcoded to 5 minutes, which causes HPAs to receive no metrics when using sparse metrics (e.g. exported every 10 minutes).

Problem

The adapter queries Cloud Monitoring with a fixed 5-minute window ([now - 5 min, now]). Metrics exported less frequently often fall outside this window, so the External Metrics API returns empty and HPAs cannot scale.

Relevant code:

• adapter.go L250
• query_builder.go (startTime := endTime.Add(-t.reqWindow))

Solution

Add a configurable flag with default 5m for backward compatibility. Users with sparse metrics can set --external-metrics-request-window=1h (or similar).

Changes

• Add ExternalMetricsRequestWindow time.Duration to stackdriverAdapterServerOptions
• Set default to 5*time.Minute in serverOptions
• Add flags.DurationVar for --external-metrics-request-window
• Pass serverOptions.ExternalMetricsRequestWindow to makeProviderOrDie instead of 5*time.Minute

Testing

• Default behavior unchanged (5m window)
• With --external-metrics-request-window=1h, sparse metrics should be returned reliably

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