Add Prometheus metrics for license warnings/errors

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Problem

Currently, we expose license state metrics (coderd_license_active_users, coderd_license_limit_users, coderd_license_user_limit_enabled) but we don't have specific metrics for license warnings or errors that could help with monitoring and alerting.

Proposed Solution

Add Prometheus metrics to track license warnings and errors:

• coderd_license_warnings - Gauge (1 = warning active, 0 = no warning)
• coderd_license_errors - Gauge (1 = error active, 0 = no error)

Implementation Notes

• Extend the existing license.MetricsCollector in /enterprise/coderd/license/metricscollector.go
• Hook into the entitlement warning system that already exists (WriteEntitlementWarningHeaders)
• Consider adding labels for different types of warnings/errors

Benefits

• Better observability for license issues
• Proactive alerting on license problems
• Historical tracking of license health

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Updated Metrics Specification

Changing the approach based on feedback:

Metrics to add:

• coderd_license_warnings - Gauge (1 = warning active, 0 = no warning)
• coderd_license_errors - Gauge (1 = error active, 0 = no error)

Removed:

• coderd_license_status - Dropped as requested
• Changed from counters to gauges to show current state rather than cumulative counts

This approach provides a cleaner way to monitor current license health status and set up alerting rules.

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Note: The issue description above is outdated. The correct specification is:

Proposed Solution

Add Prometheus metrics to track license warnings and errors:

• coderd_license_warnings - Gauge (1 = warning active, 0 = no warning)
• coderd_license_errors - Gauge (1 = error active, 0 = no error)

(The original description mentioned counters and a status metric, but those have been changed per feedback)