Implement Stringer for tcpip.StatCounter

This enables formatting tcpip.Stats readably with %+v.

PiperOrigin-RevId: 228379088
Change-Id: I6a9876454a22f151ee752cf94589b4188729458f

pkg/tcpip/tcpip.go

@@ -568,6 +568,10 @@ func (s *StatCounter) IncrementBy(v uint64) {
 	atomic.AddUint64(&s.count, v)
 }
 
+func (s *StatCounter) String() string {
+	return strconv.FormatUint(s.Value(), 10)
+}
+
 // IPStats collects IP-specific stats (both v4 and v6).
 type IPStats struct {
 	// PacketsReceived is the total number of IP packets received from the link

pkg/tcpip/tcpip_test.go

@@ -15,7 +15,9 @@
 package tcpip
 
 import (
+	"fmt"
 	"net"
+	"strings"
 	"testing"
 )
 
@@ -193,3 +195,23 @@ func TestAddressString(t *testing.T) {
 		}
 	}
 }
+
+func TestStatsString(t *testing.T) {
+	got := fmt.Sprintf("%+v", Stats{}.FillIn())
+
+	matchers := []string{
+		// Print root-level stats correctly.
+		"UnknownProtocolRcvdPackets:0",
+		// Print protocol-specific stats correctly.
+		"TCP:{ActiveConnectionOpenings:0",
+	}
+
+	for _, m := range matchers {
+		if !strings.Contains(got, m) {
+			t.Errorf("string.Contains(got, %q) = false", m)
+		}
+	}
+	if t.Failed() {
+		t.Logf(`got = fmt.Sprintf("%%+v", Stats{}.FillIn()) = %q`, got)
+	}
+}