CSV export is incorrect for multiple series in the same chart/metric

Summary

CSV export is incorrect for multiple series in the same metric.

Steps to reproduce

1. Visit a dashboard with multiple metrics
2. Click on Download CSV in the "more actions".
3. Open the CSV file and check the columns

Example Project

https://gitlab.com/gitlab-org/monitor/monitor-sandbox/-/environments/711946/metrics?dashboard=.gitlab%2Fdashboards%2Fmultiple_series_for_colors.yml

For a chart that contains 2 series:

So we could expect the CSV to contain 2 columns for data. However, only one is present:

See full file: chart_metrics__8_.csv

What is the current bug behavior?

The CSV only contains a single data for a single series.

What is the expected correct behavior?

The CSV contains all the data in the chart.

Relevant logs and/or screenshots

See above

Output of checks

This bug happens on GitLab.com

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)