Add "thread" label to pprof_proto_generator

This can be used with pprof tag filtering to filter to specific tags interactively:
https://github.com/google/pprof/blob/master/doc/README.md#tag-filtering

The "thread" label is fairly well-established on the server side pprof for logging multiple threads.

Change-Id: If8cb30c80763fc6c442cdc9c662e937af64d0289
1 file changed