pprof_proto_generator: friendlier sample_types

Drop `event_` from the front of all samples, it wasn't adding signal.

Drop `_count` suffix, the units field covers this.

BUG=201369296

Change-Id: I83514ff904aab3bb835a223a877ad7348a44419b
Test: Ran test/test.py -p TestPprofProtoGenerator.\*
2 files changed