[Stack Monitoring] Metricbeat writes to .monitoring-*-8-mb indice instead of datastream after 8.0 upgrade

### Summary

In 8.x, metricbeat writes stack monitoring data to `.monitoring-{product}-8-mb` index patterns. These patterns are backed by datastream templates created by elasticsearch at startup and define the corresponding ILM and mappings necessary for SM to function.

We may have a race condition where an unmapped/unusable indice could be created instead of the expected datastream if elasticsearch didn't setup the backing template when metricbeat start writing data to `.monitoring-{product}-8-mb`.

If scenario is confirmed, we should work on a fix that prevents data from being written to the `.monitoring-{product}-8-mb` patterns until the templates are successfully setup.

This ticket tracks the investigation and resolution of that potential scenario.