log: fix formatting of big.Int (#22679)

* log: fix formatting of big.Int

The implementation of formatLogfmtBigInt had two issues: it crashed when
the number was actually large enough to hit the big integer case, and
modified the big.Int while formatting it.

* log: don't call FormatLogfmtInt64 for int16

* log: separate from decimals back, not front

Co-authored-by: Péter Szilágyi <peterke@gmail.com>

log/format_test.go

@@ -2,6 +2,7 @@ package log
 
 import (
 	"math"
+	"math/big"
 	"math/rand"
 	"testing"
 )
@@ -58,6 +59,25 @@ func TestPrettyUint64(t *testing.T) {
 	}
 }
 
+func TestPrettyBigInt(t *testing.T) {
+	tests := []struct {
+		int string
+		s   string
+	}{
+		{"111222333444555678999", "111,222,333,444,555,678,999"},
+		{"-111222333444555678999", "-111,222,333,444,555,678,999"},
+		{"11122233344455567899900", "11,122,233,344,455,567,899,900"},
+		{"-11122233344455567899900", "-11,122,233,344,455,567,899,900"},
+	}
+
+	for _, tt := range tests {
+		v, _ := new(big.Int).SetString(tt.int, 10)
+		if have := formatLogfmtBigInt(v); have != tt.s {
+			t.Errorf("invalid output %s, want %s", have, tt.s)
+		}
+	}
+}
+
 var sink string
 
 func BenchmarkPrettyInt64Logfmt(b *testing.B) {