Clarify Prometheus scrape interval related settings
Clarify Prometheus scrape interval related settings

Recommend 15s specifically, because 30s is too long to wait for metrics
to update. Besides, 2m for rate() feels like an eternity.

re rabbitmq/rabbitmq-prometheus#3

rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website