Update logs-config.md - structured_metadata

[Abuelodelanada]

According to what I was able to investigate, grafana-agent is not able to forward structured_metadata received from promtail (or another grafana-agent instance) to Loki.

https://grafana.slack.com/archives/C01050C3D8F/p1726151855977039

PR Description

Let's imagine we have this scenario:

promtail (or grafana-agent) --> grafana-agent --> loki

And Promtail has the following config in which we remove a label (filename) to add it as structured_metadata:

clients:
- tls_config:
    insecure_skip_verify: true
  url: http://agent-0.agent-endpoints.cos.svc.cluster.local:3500/loki/api/v1/push
positions:
  filename: /opt/promtail/positions.yaml
scrape_configs:
- job_name: system
  pipeline_stages:
  - drop:
      expression: .*file is a directory.*
  - structured_metadata:
      filename: filename
  - labeldrop:
    - filename
  static_configs:
  - labels:
      __path__: /bin/fake.log
      container: workload
      job: juju_cos_702ee831_flog
    targets:
    - localhost
server:
  grpc_listen_port: 9095
  http_listen_port: 9080

And these logs are received by a grafana-agent with a config like:

logs:
  configs:
  - clients:
    - tls_config:
        insecure_skip_verify: false
      url: http://192.168.1.240/cos-loki-0/loki/api/v1/push
    name: push_api_server
    scrape_configs:
    - job_name: loki
      loki_push_api:
        server:
          grpc_listen_port: 3600
          http_listen_port: 3500
      pipeline_stages:
      - drop:
          expression: .*file is a directory.*
      - structured_metadata:
          filename: filename
      - labeldrop:
        - filename
  positions_directory: /run/grafana-agent-positions

filename is not forwarded to loki as structured_metadata.

Which issue(s) this PR fixes

Notes to the Reviewer

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated

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[clayton-cornell]

@mattdurham can you do a quick validity check on this one for Agent?

[mattdurham]

Looking at the code structured metadata should come through on both json and protobuf. We have a test for it here https://github.com/grafana/agent/blob/8908735a435029f915b7a7bfda1f8b9e7a3d9cd2/internal/component/loki/source/api/internal/lokipush/push_api_server_test.go

[clayton-cornell]

@mattdurham does that mean the doc change proposed here is not accurate? IF someone having issues with structured metadata how can we resolve this?