internal/debug: use pprof goroutine writer for debug_stacks (#16892)

* debug: Use pprof goroutine writer in debug.Stacks() to ensure all goroutines are captured.

* Up to 64MB limit, previous code only captured first 1MB of goroutines.

* internal/debug: simplify stacks handler

* fix typo

* fix pointer receiver

internal/debug/api.go

@@ -21,6 +21,7 @@
 package debug
 
 import (
+	"bytes"
 	"errors"
 	"io"
 	"os"
@@ -190,9 +191,9 @@ func (*HandlerT) WriteMemProfile(file string) error {
 
 // Stacks returns a printed representation of the stacks of all goroutines.
 func (*HandlerT) Stacks() string {
-	buf := make([]byte, 1024*1024)
-	buf = buf[:runtime.Stack(buf, true)]
-	return string(buf)
+	buf := new(bytes.Buffer)
+	pprof.Lookup("goroutine").WriteTo(buf, 2)
+	return buf.String()
 }
 
 // FreeOSMemory returns unused memory to the OS.