Add per-tenant Grafana Explore URL format for alert GeneratorURL

[CharlieTLe]

Summary

• Add support for tenants to configure alert GeneratorURL to use Grafana Explore format instead of the default Prometheus /graph format
• Introduce three new per-tenant runtime config settings: ruler_alert_generator_url_format, ruler_grafana_datasource_uid, and ruler_grafana_org_id
• Add per-tenant ruler_external_url override so tenants can have different external URLs without changing the global ruler config

When ruler_alert_generator_url_format is set to grafana-explore, the GeneratorURL in fired alerts becomes a Grafana Explore link (e.g. http://grafana:3000/explore?schemaVersion=1&panes=...&orgId=1) with the alert expression and datasource UID pre-filled. Clicking "See source" on an alert in Grafana opens Explore with the correct query, datasource, and time range.

New per-tenant config options

Setting	Description	Default
ruler_external_url	Per-tenant external URL override for the ruler	"" (uses global)
ruler_alert_generator_url_format	URL format: prometheus or grafana-explore	"" (prometheus)
ruler_grafana_datasource_uid	Grafana datasource UID for Explore links	""
ruler_grafana_org_id	Grafana org ID for Explore links	1

Example runtime config

overrides:
  my-tenant:
    ruler_external_url: "http://grafana.example.com:3000"
    ruler_alert_generator_url_format: "grafana-explore"
    ruler_grafana_datasource_uid: "my-cortex-ds"
    ruler_grafana_org_id: 1

Test plan

• Unit tests for grafanaExploreLink URL generation with various expressions and special characters
• Unit tests for userExternalURL tracking per-tenant URL changes
• Unit tests for SendAlerts with custom generator URL function
• Unit tests for ruler manager factory wiring the per-tenant overrides
• Validated exporter test updated for new limit fields
• Manual e2e test: Docker Compose stack with Cortex + Grafana, always-firing alert rule, runtime config with grafana-explore format — confirmed the "See source" link opens Grafana Explore with correct datasource and query

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