runtime/pprof: background GC goroutine sample have pprof labels

[prattmic]

gcBgMarkStartWorkers starts several gcBgMarkWorker goroutines. This is called when starting the first GC. If the GC is triggered by a goroutine with pprof labels, then the GC worker goroutines will inherit the pprof labels.

cc @mknyszek @bcmills

[mknyszek]

It's worth noting that this can also happen after the first GC if GOMAXPROCS increases.

[gopherbot]

Change https://golang.org/cl/369983 mentions this issue: runtime: do not inherit labels on system goroutines

[gopherbot]

Change https://golang.org/cl/369984 mentions this issue: runtime/pprof: assert that labels never appear on unexpected samples