chore: add banner to inform about sparse ColdStart metric

Author: heitorlessa

docs/core/metrics.md

@@ -252,6 +252,8 @@ If it's a cold start invocation, this feature will:
 
 This has the advantage of keeping cold start metric separate from your application metrics, where you might have unrelated dimensions.
 
+!!! info "We do not emit 0 as a value for ColdStart metric for cost reasons. [Let us know](https://github.com/awslabs/aws-lambda-powertools-python/issues/new?assignees=&labels=feature-request%2C+triage&template=feature_request.md&title=) if you'd prefer a flag to override it"
+
 ## Advanced
 
 ### Adding metadata