ethdev: add common stats for telemetry

ethdev: add common stats for telemetry

The ethdev library now registers a telemetry command for common ethdev
statistics.

An example usage is shown below:

Connecting to /var/run/dpdk/rte/dpdk_telemetry.v2
{"version": "DPDK 20.08.0-rc1", "pid": 14119, "max_output_len": 16384}
--> /ethdev/stats,0
{"/ethdev/stats": {"ipackets": 0, "opackets": 0, "ibytes": 0, "obytes": \
    0, "imissed": 0, "ierrors": 0, "oerrors": 0, "rx_nombuf": 0, \
    "q_ipackets": [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], \
    "q_opackets": [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], \
    "q_ibytes": [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], \
    "q_obytes": [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], \
    "q_errors": [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]}}

Signed-off-by: Ciara Power <ciara.power@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>