metric: Add gro aggregation ratio to network performance metrics

GRO aggregation is done early in the receive data flow. This patch
use napi_greo_receive related ftrace event and net_receive_skb
ftrace event to calculate gro aggregation ratio as a measure of
packet aggregation efficiency.

Bug: 215086208

Test: <trace_processor_shell> --run-metrics android_netperf \
      <perfetto_trace>
Test: tools/diff_test_trace_processor.py <trace_processor_shell> \
      --trace-filter='netperf_metric'
Change-Id: I981e271ae16646adf646d0c042b252b75bb4054f