PMM looks for pt-heartbeat in a non-standard location

I discussed this first in #pmm slack, but now a customer has mentioned it as well.

mysqld_exporter ... --collect.heartbeat defaults to looking at `heartbeat`.`heartbeat` for pt-heartbeat data. It is not possible for the user to change this parameter for the exporter in PMM.

https://github.com/percona/mysqld_exporter/blob/main/collector/heartbeat.go#L40-L51

Please consider changing the default database of the exporter to match the typical default database for most PT tools, which is `percona`.