Clarify Prometheus plugin config options required for each metric group in output example

> Q: Which version of kong is this available: kong_data_plane_config_hash Config hash value of the data plane kong_data_plane_last_seen Last time data plane contacted control plane
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> A: The metrics kong_data_plane_config_hash (Config hash value of the data plane) and kong_data_plane_last_seen (Last time data plane contacted control plane) are part of the Prometheus plugin's output. However, the knowledge sources do not explicitly state the exact Kong Gateway version in which these metrics became available.
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> The provided example output includes these metrics, but it does not specify the version. Therefore, based on the available information, I cannot confidently state the specific Kong Gateway version where these metrics were introduced. If you need the precise version, I recommend checking the Kong Gateway Prometheus plugin changelog or documentation for version-specific details [Prometheus plugin metrics output example](https://developer.konghq.com/plugins/prometheus/#metrics-output-example).

Mentioning the Kong Gateway version likely isn’t necessary. What’s missing from the Prometheus plugin documentation is a concise summary that maps each group of metrics to the corresponding config flag that must be set to `true` in the plugin configuration. This would help users understand **which metrics appear in the [[Prometheus plugin metrics output example](https://developer.konghq.com/plugins/prometheus/#metrics-output-example)](https://developer.konghq.com/plugins/prometheus/#metrics-output-example)** based on their plugin settings.