Add support for dropped fields in NetworkTracing

Dropping fields removes them from the bucketing and from the trace
output (the important part being the bucketing). The most useful is
dropping the local port which is often meaningless and can occasionally
have very high cardinality (leading to less bundling, less aggregation
and more space used by interning).

Bug: 246985031
Test: atest libnetworkstats_test
Change-Id: I2ae3289583fa7f29e60d92c58378f189a564bd81
3 files changed