Trim down beat name if it's longer than 15 characters (elastic#22550)

* Trim down beat name if it's longer than 15 characters

* add OS guard, changelog

CHANGELOG.next.asciidoc

@@ -434,6 +434,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Add interval information to `monitor` metricset in azure. {pull}22152[22152]
 - Change Session ID type from int to string {pull}22359[22359]
 - Fix filesystem types on Windows in filesystem metricset. {pull}22531[22531]
+- Fix failiures caused by custom beat names with more than 15 characters {pull}22550[22550]
 
 *Packetbeat*
 

libbeat/cmd/instance/metrics.go

@@ -43,6 +43,12 @@ func init() {
 func setupMetrics(name string) error {
 	monitoring.NewFunc(systemMetrics, "cpu", reportSystemCPUUsage, monitoring.Report)
 
+	//if the beat name is longer than 15 characters, truncate it so we don't fail process checks later on
+	// On *nix, the process name comes from /proc/PID/stat, which uses a comm value of 16 bytes, plus the null byte
+	if (runtime.GOOS == "linux" || runtime.GOOS == "darwin") && len(name) > 15 {
+		name = name[:15]
+	}
+
 	beatProcessStats = &process.Stats{
 		Procs:        []string{name},
 		EnvWhitelist: nil,