@Override
    public String toString() {
        return MoreObjects.toStringHelper(this)
                          .add("config", config())
                          .add("activePorts", activePorts())
                          .add("state", startStop)
                          .toString();
    }
Sets up gauge metric for each server certificate.
core/src/main/java/com/linecorp/armeria/server/Server.java