Allow to set metric_id_prefix per endpoint

[pilhuhn]

The global agent config has

metric_id_prefix: pod/${POD:uid}/custom/

I think it would be good if that could be overwritten on a per endpoint basis

[jmazzitelli]

This should be easy to implement, but is probably not something we want to implement.

We always have to be careful what a user can configure in the endpoints: section of the pod configmaps - especially if they override what the admin user configured in the agent global config. (this is why, for example, the endpoint tags section do not override tags defined in the agent global tags section - the agent global tags: section overrides the endpoint tags).

The issue here is the administrator that installs and configures the agent controls the agent global config. Anything in the endpoints section should rarely override anything in the agent global - because this would mean any John Doe Developer can install an endpoint in their pod and override the agent global config settings (and thus override what the administrator wants).

For the OpenShift case, overriding the metric_id_prefix is going to break the OpenShift console because the prefix is needed for the console to know what metric to graph on a specific page in the UI.

Unless there is an overriding use-case for why we would want something like this, we should close this issue as "will not fix".