Make node_exporters.enable_collectors configurable

[err404r]

Enhancement Proposal

There are many useful exporters, which could be very useful.
Currently it looks like this options is hardcoded, but this need to be changed:

Potentially we have a 3 sources for this value:

1. Default list with basic exporter, could have different values for lxd and physical host.
2. Relations, some charms could request to enable custom exporters for them, for example ntp charm
3. And the last one is charm config, For things like bcache statistics and conntrrack it is hard to identify if such exporters are actually needed so manual override would be helpful

Obviously each exporter should has corresponding list of alerts, but this is something for the future.
For now it would be cool to make it configurable, so we could enable collectors manually and add custom rules through cos-configurator.

[peppepetra]

One exporter that is currently required is ntp exporter. Right now on clouds using COS only (w/o cos-proxy i.e. new k8s, kf or managed apps) we are missing ntp monitoring.

[IbraAoad]

Scope for this will be limited to adding ntp exporter as part of the default list of exporters.

[err404r]

It should not be enabled by default. We often relate grafana-agent to containers and for containers ntp exporter have no value. It should be configurable. At least separate option for ntp exporter. But It's better to make additional exporters configurable. Because I we could make use of another exporters like bcache, conntrack, presssure, nvme and mdadm

[sed-i]

Is the idea to somehow augment the list here?

1. Re configuring collectors via rel data: need to be mindful of the snap overwrite known issue in the multi-principal scenario.
2. Need to make sure the corresponding alerts won't misbehave if an exporter is enabled/disabled.
3. Anything else @simskij @IbraAoad?

How would you propose implement this @err404r?