Document disabled burn-rate tiers for low reliability targets
Summary
The Service Levels guide says that Logfire omits a burn-rate tier when the reliability target is too low for that tier to fire. Logfire now behaves differently:
- Every reliability target always has all three burn-rate alerts (fast, medium and slow burn).
- A tier that cannot fire at the current target is kept but not evaluated. The target detail page shows it as Disabled with Target too low to fire, and its alert page says that the reliability target is too low for the alert to fire.
- A disabled tier does not count as an active alert.
- If the target is raised so that the tier can fire, the same alert runs again with its notification channels.
This PR updates docs/guides/web-ui/service-levels.md to describe that behavior. It gives the target thresholds at which the fast burn (about 93.06%) and medium burn (about 83.34%) tiers can fire. It also says that the channels you choose when you create a target are a starting set for its three alerts, and that you can change the channels of each alert later, which matches the setup wizard.
When to merge
This documents behavior in the next Logfire release. Merge it when that release is out.
Preview
I built the page locally with the unified docs preview (make docs-serve flow), together with #2459. The page renders, and every link on it resolves: the internal links to Pydantic Evals, Infrastructure as Code, Alerts, SQL Workbench, Live view and LLMs, the in-page anchors, and the Google SRE Workbook link. Please still check the hosted preview.