Feature Request: Add support for filttering in consul loader

[amuntean-godaddy]

I am registering the targets within the same service in Consul, and it would be convenient to have the consul loader being able to filter the targets as described in https://developer.hashicorp.com/consul/api-docs/features/filtering.

Here is an example of loader configuration I am expecting to have:

    loader:
      address: ${CONSUL_ADDRESS}
      token: ${CONSUL_HTTP_TOKEN}
      type: consul
      datacenter: dc1
      enable-metrics: true  
      services:
        - name: gnmi
          filter: Service.Meta.profile == "arista-dcs"
          config:
            username: ${SERVICE_ACCOUNT_USERNAME}
            password: ${SERVICE_ACCOUNT_PASSWORD}
            skip-verify: true
            tls-ca: /config/tls/tls.ca
            tls-cert: /config/tls/tls.crt
            tls-key: /config/tls/tls.key
            event-tags:
              assignment_group: "{{ .Meta.assignment_group }}"
              model: "{{ .Meta.model }}"
              network_environment: "{{ .Meta.network_environment }}"
              role: "{{ .Meta.role }}"
            subscriptions:
              - arista-dcs-system
              - arista-dcs-interfaces
        - name: gnmi
          filter: Service.Meta.profile == "juniper-ex"
          config:
            username: ${SERVICE_ACCOUNT_USERNAME}
            password: ${SERVICE_ACCOUNT_PASSWORD}
            skip-verify: true
            tls-ca: /config/tls/tls.ca
            tls-cert: /config/tls/tls.crt
            tls-key: /config/tls/tls.key
            event-tags:
              assignment_group: "{{ .Meta.assignment_group }}"
              model: "{{ .Meta.model }}"
              network_environment: "{{ .Meta.network_environment }}"
              role: "{{ .Meta.role }}"
            subscriptions:
              - juniper-ex-system
              - juniper-ex-interfaces 

This is how the targets are defined in consul:

[{
    'Address': 'X.X.X.X',
    'CreateIndex': 10583467,
    'Datacenter': 'us',
    'ID': '',
    'ModifyIndex': 10617416,
    'Node': 'assa1318-01.mgt.cgn1.gdg',
    'NodeMeta': { 'external-node': 'true' },
    'ServiceAddress': 'X.X.X.X',
    'ServiceConnect': {},
    'ServiceEnableTagOverride': False,
    'ServiceID': 'assa1318-01.mgt.cgn1.gdg',
    'ServiceKind': '',
    'ServiceLocality': None,
    'ServiceMeta': {
        'assignment_group': 'Networking',
        'model': 'CCS-720XP-48Y6',
        'network_environment': 'CGN1',
        'profile': 'arista-dcs',
        'role': 'Access Switch',
        'site': 'cgn1'
    },
    'ServiceName': 'gnmi',
    'ServicePort': 6030,
    'ServicePorts': None,
    'ServiceProxy': { 'Expose': {}, 'MeshGateway': {}, 'Mode': '' },
    'ServiceSocketPath': '',
    'ServiceTags': ['profile=arista-dcs'],
    'ServiceWeights': { 'Passing': 1, 'Warning': 1 },
    'TaggedAddresses': None
}]