examples/l3fwd-power: use new telemetry

examples/l3fwd-power: use new telemetry

The l3fwd-power example app now registers a stats command with
telemetry, and provides a callback function to handle formatting the
power stats.

An example usage is shown below:

Connecting to /var/run/dpdk/rte/dpdk_telemetry.v2
{"version": "DPDK 20.05.0-rc0", "pid": 63384, "max_output_len": 16384}
--> /
{"/": ["/", "/ethdev/link_status", "/ethdev/list", "/ethdev/xstats", \
    "/help", "/info", "/l3fwd-power/stats", "/rawdev/list", \
    "/rawdev/xstats"]}
--> /l3fwd-power/stats
{"/l3fwd-power/stats": {"empty_poll": 281625000, "full_poll": 0, \
    "busy_percent": 0}}

The existing stats tracking done by the app using the metrics
library is unaffected. This will still be used to ensure backward
compatibility.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Signed-off-by: Ciara Power <ciara.power@intel.com>
Reviewed-by: Keith Wiles <keith.wiles@intel.com>