util/clientmetric, logtail: log metric changes

Updates #3307

Change-Id: I1399ebd786f6ff7defe6e11c0eb651144c071574
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

logpolicy/logpolicy.go

@@ -38,6 +38,7 @@ import (
 	"tailscale.com/paths"
 	"tailscale.com/smallzstd"
 	"tailscale.com/types/logger"
+	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/racebuild"
 	"tailscale.com/util/winutil"
 	"tailscale.com/version"
@@ -500,6 +501,9 @@ func New(collection string) *Policy {
 		},
 		HTTPC: &http.Client{Transport: newLogtailTransport(logtail.DefaultHost)},
 	}
+	if collection == logtail.CollectionNode {
+		c.MetricsDelta = clientmetric.EncodeLogTailMetricsDelta
+	}
 
 	if val := getLogTarget(); val != "" {
 		log.Println("You have enabled a non-default log target. Doing without being told to by Tailscale staff or your network administrator will make getting support difficult.")