kvserver/rangefeed: add UpdateMetricsOnRangefeedDisconnectBy

This patch updates RangefeedMetricsRecorder to include
UpdateMetricsOnRangefeedDisconnectBy, which decrements rangefeed metrics by the
given argument. While the benefits remain unclear, future commits will use
this method to clean up metrics during multiple rangefeed disconnects.

Epic: none
Release note: none

pkg/kv/kvserver/rangefeed/sender_helper_test.go

@@ -32,7 +32,11 @@ func (c *testRangefeedCounter) UpdateMetricsOnRangefeedConnect() {
 }
 
 func (c *testRangefeedCounter) UpdateMetricsOnRangefeedDisconnect() {
-	c.count.Add(-1)
+	c.UpdateMetricsOnRangefeedDisconnectBy(1)
+}
+
+func (c *testRangefeedCounter) UpdateMetricsOnRangefeedDisconnectBy(num int64) {
+	c.count.Add(int32(-num))
 }
 
 func (c *testRangefeedCounter) get() int {

pkg/kv/kvserver/rangefeed/unbuffered_sender.go

@@ -19,6 +19,7 @@ import (
 type RangefeedMetricsRecorder interface {
 	UpdateMetricsOnRangefeedConnect()
 	UpdateMetricsOnRangefeedDisconnect()
+	UpdateMetricsOnRangefeedDisconnectBy(num int64)
 }
 
 // ServerStreamSender forwards MuxRangefeedEvents from UnbufferedSender to the

pkg/server/node.go

@@ -361,10 +361,16 @@ func (nm *nodeMetrics) UpdateMetricsOnRangefeedConnect() {
 	nm.ActiveMuxRangeFeed.Inc(1)
 }
 
-// UpdateOnRangefeedDisconnect decrements rangefeed metrics when a server
-// rangefeed is disconnected.
+// UpdateMetricsOnRangefeedDisconnect decrements rangefeed metrics when one
+// server rangefeed is disconnected.
 func (nm *nodeMetrics) UpdateMetricsOnRangefeedDisconnect() {
-	nm.ActiveMuxRangeFeed.Dec(1)
+	nm.UpdateMetricsOnRangefeedDisconnectBy(1)
+}
+
+// UpdateMetricsOnRangefeedDisconnectBy decrements rangefeed metrics by the
+// given num argument when there are multiple rangefeed disconnects.
+func (nm *nodeMetrics) UpdateMetricsOnRangefeedDisconnectBy(num int64) {
+	nm.ActiveMuxRangeFeed.Dec(num)
 }
 
 // A Node manages a map of stores (by store ID) for which it serves