Why Your PromQL Availability Query Returns Nothing When Services Are Healthy

When a PromQL availability query uses a ratio where the numerator filters for 5xx errors, it returns no data instead of 100% when services are healthy — because PromQL produces an empty vector when no matching series exist. The fix is the `* 0` fallback pattern: use `or metric{...} * 0` in the numerator so a zero-valued series with correct labels is always present. This avoids the pitfalls of `vector(0)` (which loses labels), `clamp_min` (which only floors existing series), and `absent()` (which doesn't preserve labels in grouped queries). The pattern generalizes to any PromQL ratio where the numerator can legitimately be empty.