runtime: do not inherit labels on system goroutines

GC background mark worker goroutines are created when the first GC is
triggered (or next GC after GOMAXPROCS increases). Since the GC can be
triggered from a user goroutine, those workers will inherit any pprof
labels from the user goroutine.

That isn't meaningful, so avoid it by excluding system goroutines from
inheriting labels.

Fixes golang#50032

Change-Id: Ib425ae561a3466007ff5deec86b9c51829ab5507
Reviewed-on: https://go-review.googlesource.com/c/go/+/369983
Reviewed-by: Austin Clements <austin@google.com>
Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/runtime/proc.go

@@ -4300,11 +4300,13 @@ func newproc1(fn *funcval, callergp *g, callerpc uintptr) *g {
 	newg.gopc = callerpc
 	newg.ancestors = saveAncestors(callergp)
 	newg.startpc = fn.fn
-	if _g_.m.curg != nil {
-		newg.labels = _g_.m.curg.labels
-	}
 	if isSystemGoroutine(newg, false) {
 		atomic.Xadd(&sched.ngsys, +1)
+	} else {
+		// Only user goroutines inherit pprof labels.
+		if _g_.m.curg != nil {
+			newg.labels = _g_.m.curg.labels
+		}
 	}
 	// Track initial transition?
 	newg.trackingSeq = uint8(fastrand())