Feature request: custom metrics: Y-axis (left or right) per metric

Having a single chart that plots 2 data sets is useful for event correlation when using numerical data. Currently you have to build two charts and eyeball the metrics across the two charts. By instead allowing users to create a single chart with one metric on the left Y axis and a second metric on the right Y axis, with a different scale, users could more meaningfully analyze a situation (e.g. fsync cumulative latency vs iops) without the guess work.

Jira issue: CRDB-29981