exporter/datadogexporter: add containers.* metrics mapping

[gbbr]

Add support for container metrics. To enable the dashboard metrics, add the following config:

docker_stats:
  metrics:
    container.network.io.usage.rx_packets:
      enabled: true
    container.network.io.usage.tx_packets:
      enabled: true
    container.cpu.usage.system:
      enabled: true
    container.memory.rss:
      enabled: true
    container.blockio.io_serviced_recursive:
      enabled: true

Closes #22149