Use $__rate_interval instead of hardcoded time interval values when using rate()

[hcoyote]

https://grafana.com/blog/2020/09/28/new-in-grafana-7.2-__rate_interval-for-prometheus-rate-queries-that-just-work/

Won't link internal thread, but we tend to use hardcoded intervals now for rates, when we should probably be using $__rate_interval

This starts to break down if the time span you're viewing is smaller than the selected interval.

A quick look shows we're hardcoding 1m/5m intervals in at least this dashboard

https://github.com/redpanda-data/observability/blob/main/grafana-dashboards/Redpanda-Ops-Dashboard.json https://github.com/redpanda-data/observability/blob/main/grafana-dashboards/Connectors.json https://github.com/redpanda-data/observability/blob/main/grafana-dashboards/Kafka-Consumer-Metrics.json https://github.com/redpanda-data/observability/blob/main/grafana-dashboards/Kafka-Producer-Metrics.json https://github.com/redpanda-data/observability/blob/main/grafana-dashboards/Kafka-Topic-Metrics.json