[receiver/prometheusreceiver] Fix retrieval of aggregated metrics with no job/instance label

[bacherfl]

Description: This PR fixes the retrieval of metrics where either the job or instance label is missing, and honor_labels is set to true. This can be the case for aggregated metrics coming from a federate endpoint. This PR introduces a fallback to using the job/instance labels from the scrape config for such metrics.

Link to tracking Issue: Fixes #32555

Testing:

• Added a Unit test
• Verified using the following config:

receivers:
  prometheus:
    config:
      scrape_configs:
        - job_name: 'federate'
          scrape_interval: 10s
          honor_labels: true
          params:
            'match[]':
              - '{__name__="cluster:node_cpu:sum_rate5m"}'
          metrics_path: '/federate'
          static_configs:
            - targets:
                - "localhost:9090"
exporters:
  debug:
    verbosity: detailed
  otlphttp:
    endpoint: ${env:OTLP_ENDPOINT}

  pipelines:
    metrics:
      receivers: [otlp,prometheus]
      exporters: [otlphttp, debug]

This was tested on a kind K8s cluster running the prometheus operator, with a port forward for the prometheus-k8s service created by the prometheus operator (therefore the localhost:9090 address in the target).