Generate an internal id when the id is not defined by the connector

[NassimBtk]

Is this id required when we discover and collect only one instance?

@NassimBtk There will be cases where we don't want an id attribute, because there is only 1 instance.

Idea: we could consider that if id attribute is not set, then there is a single instance and it's "internal ID" is something like "_singleton" or just the name of the monitor type (in this example: memory?)

Originally posted by @bertysentry in sentrysoftware/metricshub-community-connectors#61 (comment)

[NassimBtk]

We be done through: #341 #342 #343