How to set a logLevel for Elasticsearch and Kibana CustomResource?

Elastic Orchestration Elastic Cloud on Kubernetes (ECK)
2

You can use the configuration options provided by Elasticsearch and Kibana. The easiest option would be to use the config field to modify elasticsearch.yaml or kibana.yaml as follows:

---
apiVersion: elasticsearch.k8s.elastic.co/v1
kind: Elasticsearch
metadata:
  name: hulk
spec:
  version: 7.8.0
  nodeSets:
  - name: default
    count: 3
    config:
      logger.org.elasticsearch: warn
---
apiVersion: kibana.k8s.elastic.co/v1
kind: Kibana
metadata:
  name: hulk
spec:
  version: 7.8.0
  count: 1
  elasticsearchRef:
    name: hulk
  config:
    logging.quiet: true

You can also use environment variables if you prefer that approach:

---
apiVersion: elasticsearch.k8s.elastic.co/v1
kind: Elasticsearch
metadata:
  name: hulk
spec:
  version: 7.8.0
  nodeSets:
  - name: default
    count: 3
    podTemplate:
      spec:
        containers:
          - name: elasticsearch
            env:
              - name: logger.org.elasticsearch
                value: warn
---
apiVersion: kibana.k8s.elastic.co/v1
kind: Kibana
metadata:
  name: hulk
spec:
  version: 7.8.0
  count: 1
  elasticsearchRef:
    name: hulk
  podTemplate:
    spec:
      containers:
        - name: kibana
          env:
            - name: LOGGING_QUIET
              value: "true"

If you need more control, you can also craft your own log4j configuration file as described in Log4j Configuration.