Thank you so much @stephenb , that is a good catch!
Now my error becomes
{"level":"debug","time":"2021-08-08T17:00:09Z","message":"request failed: sending event request failed: Post "https://hm-apm-apm-http.elastic:8200/intake/v2/events\": x509: certificate signed by unknown authority (next request in ~36s)"}
As this is a self signed certificate, I found this answer: X509: certificate signed by unknown authority - #2 by jsoriano
As I am using Elastic Cloud on Kubernetes (ECK) way to install, I am not sure how to change the config.
I also tried to change to
ELASTIC_APM_VERIFY_SERVER_CERT: "false"
for my Go app. However, just like the doc says, if I am using ELASTIC_APM_SERVER_CERT, the ELASTIC_APM_VERIFY_SERVER_CERT will be ignored.
Any guide? Thanks!