Add guardrail cutoff to metric-sync API (#483)

Author: MaloChevillotte

docs/data-management/certified-metrics/eppo-schema.md

@@ -43,6 +43,7 @@ Each certified metric yaml file can also define one or more metrics (either [sim
 | `metric_display_style` <br></br> (optional) | How to display the metric, either `decimal` or `percent` (default is `decimal`) | `decimal`| 
 | `minimum_detectable_effect` <br></br> (optional)| The default [MDE](/statistics/sample-size-calculator/mde#what-is-a-minimum-detectable-effect-mde) for the metric | `0.02` | 
 | `reference_url` <br></br> (optional)| An optional URL to link to in the Eppo UI | `github.com/.../<my_metric>` | 
+| `guardrail_cutoff` <br></br> (optional)| A Guardrail cutoff value for a metric, as a decimal representing a percentage. If a metric is expected to increase, this value should be negative, to warn when the metric is decreasing by more than this value. If a metric is expected to decrease, this value should be positive, to warn when the metric is increasing by more than this value. | `-0.05` | 
 
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