Telemetry for CoS ingress packet drop accounting

Introduces telemetry support to account for packets that are dropped because of ingress port congestion. Counters for priority flow control (PFC), explicit congestion notification (ECN), and ingress drops are exported using the sensor /junos/system/linecard/interface/traffic. Counters for PFC, ECN, and ingress drops are also exported using OpenConfig sensor /interfaces/interface/. Priority group (PG) buffer utilization is exported using the sensor /junos/system/linecard/qmon-sw/.