So, after upgrade to 1.8.0 got below error in metricbeat logs of fleet-server
{"log.level":"error","@timestamp":"2021-09-23T18:51:57.519Z","log.logger":"publisher_pipeline_output","log.origin":{"file.name":"pipeline/output.go","file.line":154},"message":"Failed to connect to backoff(elasticsearch(https://elasticsearch-es-coordinator-nodes-headless.logging.svc.cluster.local:9200)): Get \"https://elasticsearch-es-coordinator-nodes-headless.logging.svc.cluster.local:9200\": x509: certificate is valid for elasticsearch-es-http, elasticsearch-es-http:9200, elasticsearch-es-http.logging.svc, elasticsearch-es-http.logging.svc.cluster.local, elasticsearch-es-coordinator-nodes, elasticsearch-es-coordinator-nodes:9200, elasticsearch-es-coordinator-nodes.logging.svc, elasticsearch-es-coordinator-nodes.logging.svc.cluster.local, not elasticsearch-es-coordinator-nodes-headless.logging.svc.cluster.local","service.name":"metricbeat","ecs.version":"1.6.0"}
{"log.level":"info","@timestamp":"2021-09-23T18:51:57.519Z","log.logger":"publisher_pipeline_output","log.origin":{"file.name":"pipeline/output.go","file.line":145},"message":"Attempting to reconnect to backoff(elasticsearch(https://elasticsearch-es-coordinator-nodes-headless.logging.svc.cluster.local:9200)) with 31 reconnect attempt(s)","service.name":"metricbeat","ecs.version":"1.6.0"}
{"log.level":"info","@timestamp":"2021-09-23T18:51:57.520Z","log.logger":"publisher","log.origin":{"file.name":"pipeline/retry.go","file.line":219},"message":"retryer: send unwait signal to consumer","service.name":"metricbeat","ecs.version":"1.6.0"}
{"log.level":"info","@timestamp":"2021-09-23T18:51:57.520Z","log.logger":"publisher","log.origin":{"file.name":"pipeline/retry.go","file.line":223},"message":" done","service.name":"metricbeat","ecs.version":"1.6.0"}
{"log.level":"error","@timestamp":"2021-09-23T18:51:57.527Z","log.logger":"esclientleg","log.origin":{"file.name":"transport/logging.go","file.line":37},"message":"Error dialing x509: certificate is valid for elasticsearch-es-http, elasticsearch-es-http:9200, elasticsearch-es-http.logging.svc, elasticsearch-es-http.logging.svc.cluster.local, elasticsearch-es-coordinator-nodes, elasticsearch-es-coordinator-nodes:9200, elasticsearch-es-coordinator-nodes.logging.svc, elasticsearch-es-coordinator-nodes.logging.svc.cluster.local, not elasticsearch-es-coordinator-nodes-headless.logging.svc.cluster.local","service.name":"metricbeat","network":"tcp","address":"elasticsearch-es-coordinator-nodes-headless.logging.svc.cluster.local:9200","ecs.version":"1.6.0"}
Updating the custom certificates made the trick, it works now