telemetry: update metrics before sending stats

This patch adds functionality to update the statistics in
the metrics library with values from the ethdev stats.

Values need to be updated before they are encoded into a JSON
message and sent to the client that requested them. The JSON encoding
will be added in a subsequent patch.

Signed-off-by: Ciara Power <ciara.power@intel.com>
Signed-off-by: Brian Archbold <brian.archbold@intel.com>
Signed-off-by: Kevin Laatz <kevin.laatz@intel.com>
Acked-by: Harry van Haaren <harry.van.haaren@intel.com>