[Docker] Stop returning errors when there are no metric values

[zmoog]

Proposed commit message

Stop returning errors when there are no metric values for the Docker memory metricset.

The docker metricset returns an error when there are no memory metric values available. This condition can happen when there are no running containers on Docker.

When no containers are running, the metricset returns an error at every collection, creating noise.

Notes for reviewers

• Is there a use case when we should return an error instead?
• Should I add a logger to log a debug message saying "no memory stats data available"?

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @zmoog? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[zmoog]

@fearful-symmetry, do you see any issue in replacing the error with a debug log message? I see you added the error years ago.

[zmoog]

Here is an example of log message:

{
    "log.level": "debug",
    "@timestamp": "2024-06-05T22:48:55.082+0200",
    "log.logger": "memory",
    "log.origin": {
        "function": "github.com/elastic/beats/v7/metricbeat/module/docker/memory.(*MetricSet).Fetch",
        "file.name": "memory/memory.go",
        "file.line": 84
    },
    "message": "No memory stats data available",
    "service.name": "metricbeat",
    "ecs.version": "1.6.0"
}

It looks good, but the log.logger value "memory" is not great; it requires checking the log.origin.function value to learn it's part of the docker module.

What's the expected format in Baats?

[faec]

Nit about the logger name, otherwise looks good