Fix queue time metric in Indexed Gossip Queue of networking dashboard

**_{} Give each per-topic dashboard series a distinct legend**

Once multiple gossip topics report samples, the query in `dashboards/lodestar_networking.json:4922-4924` returns one series per `topic`, but its fixed `legendFormat` names every series `queue_time`. Operators viewing the standard dashboard therefore cannot identify which line belongs to `data_column_sidecar` or another topic, undermining the diagnostic purpose of this metric. Use the topic label in the legend, or explicitly aggregate or filter the query.

AGENTS.md reference: [AGENTS.md:L189-L196](https://github.com/ChainSafe/lodestar/blob/142db98cf236e0a957e8f293c398f98587a59340/AGENTS.md#L189-L196)

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_Originally posted by @chatgpt-codex-connector[bot] in https://github.com/ChainSafe/lodestar/pull/9940#discussion_r3892194451_