Add note about Thanos Query to monitoring docs

Why is this change being made?

I was looking for a way to query the Container Registry Prometheus metrics and was unable to find them in the clusters listed in the monitoring section of the Engineering handbook.

After posting a question in Slack, @igorwwwwwwwwwwwwwwwwwwww pointed me to Thanos Query (thanks!). I was unable to find this documented somewhere, so this MR is a proposal to add a note about Thanos Query to the handbook monitoring section.

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