Add tabular chart screen shot to Prometheus docs

What does this MR do?

As the default look of the chart legends in Operations > Metrics changed from inline to table, we need to also reflect this change in the documentation screenshots.

Add updated screenshot in the documentation.

Suggested spot: https://docs.gitlab.com/ee/user/project/integrations/prometheus.html#getting-metrics-to-display-on-the-metrics-dashboard

Before (as of v12.8)	After (as of v13.1)

Related issues

#211340 (closed)

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