Add comments to pprof_proto_generator

These show up in some pprof viewers.

There's still a bunch more metadata we could add in here, but this is a decent start.

BUG=202799418

Change-Id: Ia59fa120de7d2f14cefd58820587e7eda6e7f0ef
Test: Ran test/test.py -p TestPprofProtoGenerator.\*
2 files changed