filebeat/input/{tcp,udp}: use correct address list in udp (#35996)

Also add a check in the common path to check that the addr slice is the
same length as the undefined addr slice, returning an error on mismatch
rather than allowing the panic.

(cherry picked from commit e741a6d)

CHANGELOG.next.asciidoc

@@ -144,6 +144,7 @@ https://github.com/elastic/beats/compare/v8.8.1\...main[Check the HEAD diff]
 - Fix metric collection in GCPPubSub input. {pull}35773[35773]
 - Fix end point deregistration in http_endpoint input. {issue}35899[35899] {pull}35903[35903]
 - Fix duplicate ID panic in filestream metrics. {issue}35964[35964] {pull}35972[35972]
+- Improve error reporting and fix IPv6 handling of TCP and UDP metric collection. {pull}35996[35996]
 
 *Heartbeat*
 

filebeat/input/tcp/input.go

@@ -338,6 +338,9 @@ func procNetTCP(path string, addr []string, hasUnspecified bool, addrIsUnspecifi
 	if len(addr) == 0 {
 		return 0, nil
 	}
+	if len(addr) != len(addrIsUnspecified) {
+		return 0, errors.New("mismatched address/unspecified lists: please report this")
+	}
 	b, err := os.ReadFile(path)
 	if err != nil {
 		return 0, err

filebeat/input/udp/input.go

@@ -272,7 +272,7 @@ func (m *inputMetrics) poll(addr, addr6 []string, each time.Duration, log *logp.
 	if err != nil {
 		log.Warnf("failed to get initial udp stats from /proc: %v", err)
 	}
-	rx6, drops6, err := procNetUDP("/proc/net/udp6", addr, hasUnspecified6, addrIsUnspecified6)
+	rx6, drops6, err := procNetUDP("/proc/net/udp6", addr6, hasUnspecified6, addrIsUnspecified6)
 	if err != nil {
 		log.Warnf("failed to get initial udp6 stats from /proc: %v", err)
 	}
@@ -288,7 +288,7 @@ func (m *inputMetrics) poll(addr, addr6 []string, each time.Duration, log *logp.
 				log.Warnf("failed to get udp stats from /proc: %v", err)
 				continue
 			}
-			rx6, drops6, err := procNetUDP("/proc/net/udp6", addr, hasUnspecified6, addrIsUnspecified6)
+			rx6, drops6, err := procNetUDP("/proc/net/udp6", addr6, hasUnspecified6, addrIsUnspecified6)
 			if err != nil {
 				log.Warnf("failed to get udp6 stats from /proc: %v", err)
 				continue
@@ -333,6 +333,9 @@ func procNetUDP(path string, addr []string, hasUnspecified bool, addrIsUnspecifi
 	if len(addr) == 0 {
 		return 0, 0, nil
 	}
+	if len(addr) != len(addrIsUnspecified) {
+		return 0, 0, errors.New("mismatched address/unspecified lists: please report this")
+	}
 	b, err := os.ReadFile(path)
 	if err != nil {
 		return 0, 0, err