Add pid & tid labels to pprof_proto_generator.py

Uses precedent set by perf_data_converter:

https://github.com/google/perf_data_converter/blob/e8a65906a406fedcc0e78b75f95de16f2d99eecc/src/perf_data_converter.h#L57

Example on a Google Maps profile:

```
(pprof) tags
 pid: Total 103383248686.0
       103383248686.0 (  100%): 14330
 tid: Total 103383248686.0
      22119496619.0 (21.40%): 14364
      13358985234.0 (12.92%): 14351
       7386956769.0 ( 7.15%): 14352
       5422475643.0 ( 5.25%): 14362
       5307503967.0 ( 5.13%): 14346
       5100700923.0 ( 4.93%): 14330
       4790949129.0 ( 4.63%): 14370
       3146972395.0 ( 3.04%): 14367
       2886594038.0 ( 2.79%): 14384
       2629996668.0 ( 2.54%): 14398
       2614501772.0 ( 2.53%): 14401
       2317441688.0 ( 2.24%): 14460
       2268259602.0 ( 2.19%): 14408
       2129146149.0 ( 2.06%): 14372
       1862608311.0 ( 1.80%): 14459
       [...]
```

BUG=200992206
Test: Ran test/test.py -p TestPprofProtoGenerator.\*
Change-Id: Icbc1503ca05c46387eff3f6799f0c16df4b78001

Change-Id: I7aecbb9a520aa5f6b06f96a844050a2189d14a52
2 files changed