Handle NaNs in prometheus metrics

[jsoriano]

Prometheus metrics can contain NaN/+Inf/-Inf values, they fail to be serialized for outputs.

Seen at least with file and kafka outputs with metricbeat 6.6.0.

[pawelprazak]

Can confirm for v6.7.2, v7.0.1, v7.1.0 with this settings:

    - module: prometheus
      period: 10s
      host: ${NODE_NAME}
      hosts: ["prometheus.monitoring.svc.cluster.local:9090"]
      namespace: monitoring
      metrics_path: '/federate'
      query:
        'match[]': '{__name__!=""}'