[DOC] Update overrides configuration for metrics-generator

[knylander-grafana]

Update the overrides configuration doc to make sure that the config blog is update for metrics-generator.

Reference: #3462

Deployed with Helm on K8s. See description and comments in #3462 for details.

Example override config that works:

  overrides.yaml: |

    overrides:
      tracing-apps-app-dev:
        ingestion:
          rate_strategy: local
          rate_limit_bytes: 15000000
          burst_size_bytes: 20000000
          max_traces_per_user: 10000
        read:
          max_bytes_per_tag_values_query: 5000000
        global:
          max_bytes_per_trace: 1500000
        metrics_generator:
          processors: [service-graphs, span-metrics]
          remote_write_headers:
            X-Scope-OrgID: prometheus-apps-app-dev

Questions to answer:

• Which pages need to be updated? Helm doc and metrics-generator and overrides doc in Configuration section? Or other places?

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