Hm, let me take a look at this. I know @Detho was able to pull this one out some time ago. It seemed as if it was a VM-scope metric rather then host one. Maybe he will be able to share his method in the meantime.

API v1 has stats as part of the VMs

[…]
  "entities": "
    {
      "vmId": “xxxxxxxxxxxxxx”,
      "uuid": “xxxxxxx”,
      "powerState": "off",
      "vmName": “abcd”,
      "guestOperatingSystem": null,
      "ipAddresses": n],
      "hypervisorType": "kKvm",
      "hostName": null,
      "hostId": null,
      "hostUuid": null,
      "containerIds":
        “xxxxxx8"
      u…]
      "stats": {
        "hypervisor_avg_io_latency_usecs": "-1",
        "num_read_iops": "-1",
        "hypervisor_write_io_bandwidth_kBps": "-1",
        "timespan_usecs": "-1",
        "controller_num_read_iops": "-1",
        "controller.storage_tier.ssd.usage_bytes": "97837056",
        "read_io_ppm": "-1",
        "controller_num_iops": "-1",
        "hypervisor_memory_assigned_bytes": "-1",
        "total_read_io_time_usecs": "-1",
        "controller_total_read_io_time_usecs": "0",
        "controller.storage_tier.ssd.configured_pinned_bytes": "0",
        "hypervisor_num_io": "-1",
        "controller_total_transformed_usage_bytes": "0",
        "hypervisor_cpu_usage_ppm": "0",
        "controller_num_write_io": "-1",
        "avg_read_io_latency_usecs": "-1",
        "guest.memory_swapped_in_bytes": "-1",
        "controller_total_io_time_usecs": "0",
        "memory_usage_ppm": "0",
        "controller_total_read_io_size_kbytes": "0",
        "controller_num_seq_io": "-1",
        "controller_read_io_ppm": "-1",
        "controller_total_io_size_kbytes": "0",
        "hypervisor.cpu_ready_time_ppm": "-1",
        "controller_num_io": "0",
        "hypervisor_avg_read_io_latency_usecs": "-1",
        "num_write_iops": "-1",
        "controller_num_random_io": "0",
        "num_iops": "-1",
        "guest.memory_usage_ppm": "0",
        "hypervisor_num_read_io": "-1",
        "hypervisor_total_read_io_time_usecs": "-1",
        "controller_avg_io_latency_usecs": "-1",
        "num_io": "-1",
        "controller_num_read_io": "0",
        "hypervisor_num_write_io": "-1",
        "controller_seq_io_ppm": "-1",
        "guest.memory_usage_bytes": "0",
        "controller_read_io_bandwidth_kBps": "-1",
        "controller_io_bandwidth_kBps": "-1",
        "hypervisor_num_received_bytes": "0",
        "hypervisor_timespan_usecs": "-1",
        "hypervisor_num_write_iops": "-1",
        "total_read_io_size_kbytes": "-1",
        "hypervisor_total_io_size_kbytes": "-1",
        "avg_io_latency_usecs": "-1",
        "hypervisor_num_read_iops": "-1",
        "hypervisor_swap_in_rate_kBps": "-1",
        "controller_write_io_bandwidth_kBps": "-1",
        "controller_write_io_ppm": "-1",
        "controller_user_bytes": "9475319296",
        "hypervisor_avg_write_io_latency_usecs": "-1",
        "hypervisor_num_transmitted_bytes": "0",
        "hypervisor_total_read_io_size_kbytes": "-1",
        "read_io_bandwidth_kBps": "-1",
        "guest.memory_swapped_out_bytes": "-1",
        "hypervisor_memory_usage_ppm": "0",
        "hypervisor_num_iops": "-1",
        "hypervisor_io_bandwidth_kBps": "-1",
        "controller_num_write_iops": "-1",
        "total_io_time_usecs": "-1",
        "controller_random_io_ppm": "-1",
        "controller.storage_tier.das-sata.usage_bytes": "13925916672",
        "controller_avg_read_io_size_kbytes": "-1",
        "hypervisor_swap_out_rate_kBps": "-1",
        "total_transformed_usage_bytes": "-1",
        "avg_write_io_latency_usecs": "-1",
        "num_read_io": "-1",
        "write_io_bandwidth_kBps": "-1",
        "hypervisor_read_io_bandwidth_kBps": "-1",
        "hypervisor_consumed_memory_bytes": "0",
        "random_io_ppm": "-1",
        "total_untransformed_usage_bytes": "-1",
        "hypervisor_total_io_time_usecs": "-1",
        "num_random_io": "-1",
        "controller_avg_write_io_size_kbytes": "-1",
        "controller_avg_read_io_latency_usecs": "-1",
        "controller.storage_tier.das-sata.configured_pinned_bytes": "0",
        "num_write_io": "-1",
        "total_io_size_kbytes": "-1",
        "controller.storage_tier.cloud.usage_bytes": "0",
        "io_bandwidth_kBps": "-1",
        "controller_timespan_usecs": "0",
        "num_seq_io": "-1",
        "seq_io_ppm": "-1",
        "write_io_ppm": "-1",
        "controller_avg_write_io_latency_usecs": "-1"
      },

An article with examples Getting VM Performance Metrics via API as well.

Are there plans to have all the metrics under a single API version?

As per the documentation ‘Users of the v1 API are encouraged to migrate to v2’.

So when are the metrics available via v1 also be available via v2 and v3?

I have not heard of intention to merge the API versions, unfortunately. I can see how it would be handy.