fix(dashboards): use rate_interval instead of interval

[lucabello]

Issue

Closes #274.

Solution

Use $__rate_interval instead of $__interval.

[mmkay]

One thing to potentially consider here: https://hoelz.ro/blog/use-caution-when-using-rate_interval-along-with-increase

This might change the values scale visible in the dashboards. Might be surprising to some?

I wonder if we shouldn't change the variable only in the panels that use rate(.

[lucabello]

You're right, that replace was too lazy :) Now we only changed to $__rate_interval in rate() queries.