all: fix six tests that failed with -count=2

Avery found a bunch of tests that fail with -count=2.

Updates tailscale/corp#40176 (tracks making our CI detect them)

Change-Id: Ie3e4398070dd92e4fe0146badddf1254749cca20
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Co-authored-by: Avery Pennarun <apenwarr@tailscale.com>

util/clientmetric/clientmetric.go

@@ -22,6 +22,7 @@ import (
 
 	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/util/set"
+	"tailscale.com/util/testenv"
 )
 
 var (
@@ -452,6 +453,24 @@ func (b *deltaEncBuf) writeHexVarint(v int64) {
 	b.buf.Write(hexBuf)
 }
 
+// ResetForTest resets all client metric values to zero.
+// It panics if not in a test or if called from a parallel test.
+func ResetForTest(t testenv.TB) {
+	if !testenv.InTest() {
+		panic("clientmetric.ResetForTest called outside a test")
+	}
+	if testenv.InParallelTest(t) {
+		panic("clientmetric.ResetForTest called from a parallel test")
+	}
+	mu.Lock()
+	defer mu.Unlock()
+	for _, m := range metrics {
+		if m.v != nil {
+			atomic.StoreInt64(m.v, 0)
+		}
+	}
+}
+
 var TestHooks testHooks
 
 type testHooks struct{}

util/clientmetric/omit.go

@@ -30,3 +30,5 @@ func NewCounter(string) *Metric                   { return &zeroMetric }
 func NewGauge(string) *Metric                     { return &zeroMetric }
 func NewAggregateCounter(string) *Metric          { return &zeroMetric }
 func NewCounterFunc(string, func() int64) *Metric { return &zeroMetric }
+
+func ResetForTest(any) {}