feat: Provide capability to define default custom labels

[tomkerkhove]

Fixes #1608

Example

Assuming this configuration:

version: v1
azureMetadata:
  tenantId: c8819874-9e56-4e3f-b1a8-1c0325138f27
  subscriptionId: 0f9d7fea-99e8-4768-8672-06a28514f77e
  resourceGroupName: promitor
metricDefaults:
  aggregation:
    interval: 00:05:00
  labels:
    geo: china
    environment: dev
  scraping:
    # Every minute
    schedule: "0 * * ? * *"
metrics:
- name: promitor_demo_function_memory_discovery
  description: "Average memory for an Azure Function App"
  resourceType: FunctionApp
  azureMetricConfiguration:
    metricName: MemoryWorkingSet
    aggregation:
      type: Average
  resourceDiscoveryGroups:
  - name: function-app-landscape
- name: promitor_demo_servicebus_messagecount_discovered
  description: "Average percentage of memory usage on an Azure App Plan"
  resourceType: ServiceBusNamespace
  labels:
    geo: europe
    app: promitor
  azureMetricConfiguration:
    metricName: ActiveMessages
    aggregation:
      type: Average
  resourceDiscoveryGroups:
  - name: service-bus-landscape

It generates the following input:

# HELP promitor_demo_function_memory_discovery Average memory for an Azure Function App
# TYPE promitor_demo_function_memory_discovery gauge
promitor_demo_function_memory_discovery{resource_group="promitor-sources",subscription_id="0f9d7fea-99e8-4768-8672-06a28514f77e",resource_uri="subscriptions/0f9d7fea-99e8-4768-8672-06a28514f77e/resourceGroups/promitor-sources/providers/Microsoft.Web/sites/promitor-function-app",instance_name="promitor-function-app",slot_name="production",geo="china",environment="dev"} 40055547.66101695 1619636185372
# HELP promitor_demo_servicebus_messagecount_discovered Average percentage of memory usage on an Azure App Plan
# TYPE promitor_demo_servicebus_messagecount_discovered gauge
promitor_demo_servicebus_messagecount_discovered{resource_group="keda-demos",subscription_id="0f9d7fea-99e8-4768-8672-06a28514f77e",resource_uri="subscriptions/0f9d7fea-99e8-4768-8672-06a28514f77e/resourceGroups/keda-demos/providers/Microsoft.ServiceBus/namespaces/keda-demos",instance_name="keda-demos",entity_name="orders",geo="europe",app="promitor",environment="dev"} 0 1619636186009

Hence that the metric-level label for geo wins over the default one.