Config loggers by labels

[SimonRichardson]

To support labels from Juju, we would like to be able to configure loggers by their labels. So if a logger has a label of a model namespace (UUID) then searching from the root module to set the level of a module should be possible.

Label matching can be applied to the loggers by providing a set of labels to the ConfigureLoggers method. If a logger has a label that matches the provided labels, then the logger will be configured with the provided level. If the logger does not have a label that matches the provided labels, then the logger will not be configured. No labels will configure all loggers in the specification.

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In addition, I've removed the attachment of the dynamic labels to the logger type. There isn't a usage from Juju's perspective to ever need them. Loggo is a hierarchical model, where loggers inherit from their parents. The original label concept didn't follow that concept. For example, module names walk their parents for an effective level if a level hasn't been set. We do the inverse for labels, where the parent sets the child labels on construction (labels are immutable atm), but they do follow the same concept; child-parent relationship.