fluent-bit: Modify filter not working with Record Accessors

Bug Report

When using the modify filter to copy nested fields using Record Accessors, the filter fails to find the fields.

To Reproduce

• Configuration:

[FILTER]
    Name                modify
    Match               kube.*
    Copy                $kubernetes['labels']['app']    application
    Copy                $kubernetes['labels']['app.kubernetes.io/name'] application
    Copy                $kubernetes['container_name']   application
    Remove              $kubernetes['pod_id']
    Remove              $kubernetes['docker_id']
    Remove              $kubernetes['container_hash']

• Fluent Bit debug log

[2022/01/21 14:54:40] [debug] [filter:modify:modify.4] Rule COPY $kubernetes['labels']['app'] TO application : No keys matching $kubernetes['labels']['app'] found, not applying rule
[2022/01/21 14:54:40] [debug] [filter:modify:modify.4] Rule COPY $kubernetes['labels']['app.kubernetes.io/name'] TO application : No keys matching $kubernetes['labels']['app.kubernetes.io/name'] found, not applying rule
[2022/01/21 14:54:40] [debug] [filter:modify:modify.4] Rule COPY $kubernetes['container_name'] TO application : No keys matching $kubernetes['container_name'] found, not applying rule

• Output (sent to aws Cloudwatch)

{
    "log": "[14:55:24 INF]  REDACTED",
    "stream": "stdout",
    "time": "2022-01-21T14:55:24.532788648Z",
    "kubernetes": {
        "pod_name": "REDACTED",
        "namespace_name": "REDACTED",
        "pod_id": "723efe40-122a-4b93-960c-f79b09dc583f",
        "labels": {
            "app": "REDACTED",
            "controller-uid": "618de15b-8c23-47a2-8ac8-34cc1d59122d",
            "job-name": "REDACTED",
            "release": "REDACTED"
        },
        "annotations": {
            "kubernetes.io/psp": "eks.privileged"
        },
        "host": "REDACTED",
        "container_name": "REDACTED",
        "docker_id": "59a6665e0e27623e475a49a382e070c63c72b8673e6debcdc63444a455ecb6a9",
        "container_hash": "REDACTED",
        "container_image": "REDACTED"
    }
}

• Steps to reproduce the problem:

Expected behavior ‘application’ field should be added to the log structure with the value taken from one of the 3 found fields. $kubernetes[‘pod_id’], $kubernetes[‘docker_id’], $kubernetes[‘container_hash’] should be removed from the log structure.

Your Environment

• Version used: Latest
• Configuration:

[FILTER]
    Name                kubernetes
    Match               kube.*
    Merge_Log           On
    Keep_Log            Off
    K8S-Logging.Parser  On
    K8S-Logging.Exclude On

[FILTER]
    Name                expect
    Match               kube.*
    key_exists          kubernetes
    key_exists          log
    action              exit

[FILTER]
    Name                grep
    Match               kube.*
    Exclude             $kubernetes['namespace_name']   kube-system

[FILTER]
    Name                stdout
    Match               kube.*

[FILTER]
    Name                modify
    Match               kube.*
    Copy                $kubernetes['labels']['app']    application
    Copy                $kubernetes['labels']['app.kubernetes.io/name'] application
    Copy                $kubernetes['container_name']   application
    Remove              $kubernetes['pod_id']
    Remove              $kubernetes['docker_id']
    Remove              $kubernetes['container_hash']

• Environment name and version (e.g. Kubernetes? What version?): Kubernetes, Latest
• Operating System and version: AWS Linux
• Filters and plugins: kubernetes, expect, grep, stdout, modify, cloudwatch_logs