client/local: add method to set gauge metric to a value

The existing client metric methods only support incrementing (or
decrementing) a delta value.  This new method allows setting the metric
to a specific value.

Updates tailscale/corp#35327

Change-Id: Ia101a4a3005adb9118051b3416f5a64a4a45987d
Signed-off-by: Will Norris <will@tailscale.com>

util/clientmetric/omit.go

@@ -13,6 +13,13 @@ func (*Metric) Value() int64           { return 0 }
 func (*Metric) Register(expvarInt any) {}
 func (*Metric) UnregisterAll()         {}
 
+type MetricUpdate struct {
+	Name  string `json:"name"`
+	Type  string `json:"type"`
+	Value int    `json:"value"`
+	Op    string `json:"op"`
+}
+
 func HasPublished(string) bool            { panic("unreachable") }
 func EncodeLogTailMetricsDelta() string   { return "" }
 func WritePrometheusExpositionFormat(any) {}