log_to_metrics: allow custom namespace and subsystem

[flobz]

This PR allows to change namespace and subsystem for log_to_metrics.

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Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change

[SERVICE]
    Flush     1
    Daemon    off
    Log_Level info


[INPUT]
    Name               dummy
    Dummy              {"message":"dummy", "kubernetes":{"namespace_name": "default", "docker_id": "abc123", "pod_name": "pod1", "container_name": "mycontainer", "pod_id": "def456", "labels":{"app": "app1"}}, "duration": 20, "color": "red", "shape": "circle"}
    Tag                dummy.log
    Interval_sec               10


[FILTER]
    name               log_to_metrics
    match              dummy.log*
    tag                test_metric
    metric_mode        counter
    metric_name        count_all_dummy_messages
    metric_description This metric counts dummy messages
    metric_namespace my_ns
    metric_subsystem my_susbsystem

[OUTPUT]
    Name stdout
    Match test_metric

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• [N/A] Run local packaging test showing all targets (including any new ones) build.
• [N/A] Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

Backporting

• [N/A] Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[edsiper]

thank you !