Replace $__interval with $__rate_interval
Replace $__interval with $__rate_interval

Signed-off-by: Ryan J. Geyer <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@ryangeyer.com>

roman-vynar/mysqld_exporter,prometheus/mysqld_exporter,prometheus/mysqld_exporter,roman-vynar/mysqld_exporter