Add per-tenant alert generator URL template for customizable alert source links

[CharlieTLe]

Summary

• Replace Grafana-specific config fields with a single generic ruler_alert_generator_url_template field that accepts a Go text/template string
• Users can construct any URL format (Grafana Explore, Perses, or any metrics explorer) without Cortex needing to understand specific UI formats
• Keep per-tenant ruler_external_url override so tenants can have different external URLs without changing the global ruler config

When ruler_alert_generator_url_template is set, the ruler evaluates the template with .ExternalURL and .Expression variables to produce the alert's GeneratorURL. Built-in Go template functions like urlquery are available for URL encoding. If the template is empty, the default Prometheus /graph format is used.

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_template	Go text/template for alert generator URLs	"" (Prometheus format)

Example runtime config

overrides:
  # Grafana Explore links
  tenant-a:
    ruler_external_url: "http://grafana:3000"
    ruler_alert_generator_url_template: >-
      {{ .ExternalURL }}/explore?schemaVersion=1&panes=%7B%22default%22:%7B%22datasource%22:%22my-ds%22,%22queries%22:%5B%7B%22refId%22:%22A%22,%22expr%22:%22{{ urlquery .Expression }}%22%7D%5D,%22range%22:%7B%22from%22:%22now-1h%22,%22to%22:%22now%22%7D%7D%7D&orgId=1

  # Perses explore links
  tenant-b:
    ruler_external_url: "http://perses:8080"
    ruler_alert_generator_url_template: >-
      {{ .ExternalURL }}/explore?expr={{ urlquery .Expression }}

  # Default Prometheus /graph format (no template needed)
  tenant-c: {}

Getting-started docker-compose example

The PR includes a working docker-compose example in docs/getting-started/ with:

• Cortex with runtime config enabling per-tenant URL templates
• Grafana with per-tenant Alertmanager datasources (tenant-a, tenant-b)
• Perses v0.53.1 with explorer enabled and per-tenant Prometheus datasources
• Demo alert rules that fire immediately for both tenants
• Clicking "See source" on tenant-a alerts opens Grafana Explore; tenant-b alerts open Perses explore

Test plan

• Unit tests for executeGeneratorURLTemplate with various expressions, urlquery, and invalid templates
• Unit tests for userExternalURL tracking per-tenant URL changes
• Unit tests for SendAlerts with custom generator URL function (default + template formats)
• Template parse validation in Limits.Validate() catches invalid templates at config load time
• Validated exporter test updated for removed Grafana-specific limit fields
• Manual e2e test: Docker Compose with Cortex + Grafana + Perses, always-firing alert rules for tenant-a and tenant-b, confirmed "See source" links open the correct explorer with correct datasource and query

🤖 Generated with Claude Code

[yeya24]

@rajagopalanand Can you help take a look?

[friedrichg]

I gave a review, but I think we do not care what parameters are send to the url. We don't care about grafana org IDs

I think we should make this general enough to be used by any metrics explorer.
Maybe consider renaming the function as explore and not have any grafana

I am thinking of something that would support for example:
explore

what do you think?