util/multierr: optimize New for nil cases (#6750)

Consider the following pattern:

	err1 := foo()
	err2 := bar()
	err3 := baz()
	return multierr.New(err1, err2, err3)

If err1, err2, and err3 are all nil, then multierr.New should not allocate.
Thus, modify the logic of New to count the number of distinct error values
and allocate the exactly needed slice. This also speeds up non-empty error
situation since repeatedly growing with append is slow.

Performance:

	name         old time/op    new time/op    delta
	Empty-24       41.8ns ± 2%     6.4ns ± 1%   -84.73%  (p=0.000 n=10+10)
	NonEmpty-24     120ns ± 3%      69ns ± 1%   -42.01%  (p=0.000 n=9+10)

	name         old alloc/op   new alloc/op   delta
	Empty-24        64.0B ± 0%      0.0B       -100.00%  (p=0.000 n=10+10)
	NonEmpty-24      168B ± 0%       88B ± 0%   -47.62%  (p=0.000 n=10+10)

	name         old allocs/op  new allocs/op  delta
	Empty-24         1.00 ± 0%      0.00       -100.00%  (p=0.000 n=10+10)
	NonEmpty-24      3.00 ± 0%      2.00 ± 0%   -33.33%  (p=0.000 n=10+10)

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

util/multierr/multierr_test.go

@@ -7,6 +7,7 @@ package multierr_test
 import (
 	"errors"
 	"fmt"
+	"io"
 	"testing"
 
 	qt "github.com/frankban/quicktest"
@@ -106,3 +107,20 @@ func TestRange(t *testing.T) {
 		return x.Error() == y.Error()
 	})), want)
 }
+
+var sink error
+
+func BenchmarkEmpty(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		sink = multierr.New(nil, nil, nil, multierr.Error{})
+	}
+}
+
+func BenchmarkNonEmpty(b *testing.B) {
+	merr := multierr.New(io.ErrShortBuffer, io.ErrNoProgress)
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		sink = multierr.New(io.ErrUnexpectedEOF, merr, io.ErrClosedPipe)
+	}
+}