Stack monitoring doesn't work with a resource monitored with a custom certificate without a CA

[thbkrkr]

Stack Monitoring incorrectly assumes that if the monitored Elastic resource has TLS enabled, there is a CA to configure in the input section of the Metricbeat config. This overlooks that you can have a certificate issued by a well-known CA, so you don't always provide a CA when TLS is enabled. This makes Stack Monitoring impossible to use with a custom certificate without a CA.

Symptom: Metricbeat is in a boot loop due to an invalid configuration.

2022-01-31T23:09:16.106Z ERROR [kibana.stats] stats/stats.go:79 1 error: open 
/mnt/elastic-internal/kb-monitoring/<es-namespace>/<es-name>/certs/ca.crt: 
no such file or directory reading <nil> accessing config