Allow atoms to log fields in bytes format.

There are an increasing number of requests to log data in complex format to statsd, while the data
is not expected to be parsed or aggregated by statsd and only to be uploaded as events.

Instead of making an exception for each of these cases in a hard coded way, this CL add a feature to
annotate these field in atoms.proto and the stats-log-api-gen tool will produce byte array
interfaces for them.

Note that log_msg does not have byte array type, and only has string type, when statsd receives the
log, these fields are in string type. Only when the atom is written to proto, we will check if this
field should be bytes field and write it to protobuf in message format.

Change-Id: If53dd95c5826710c76d7fe982bf951a435dfc738
Fix: 118386797
Test: unit test & manual test