Consistent metric labelling

[Fra-nk]

Currently the labels of different components of this exporter are not aligned.

• azure_costs_eur labels are written in CamelCase, while azure_vms_allocated et al are written in snake_case.
• ResourceGroup values are lowercase, resource_group values are UPPERCASE
• azure_costs_eur has the label SubscriptionName="<literal name>", while azure_vms_allocated has the label subscription="<UUID>"