feat(experiments): add cumulative exposures chart

[jurajmajerik]

Changes

Adds a chart to display cumulative exposures from ExperimentExposuresQueryRunner. The chart shows for running experiments that use the new metrics.

Notes:

• The chart should show the full time series for the experiment's duration. However, some initial days may have no exposures. To handle this, I prefill the time series with zeroes programmatically since AFAIK achieving this directly with the query seems tricky.
• Variant colors aren’t working yet because they rely on the insight field, which is currently missing. Added a TODO in our working doc.
• Removed old CumulativeExposuresChart which was never rolled out.

How did you test this code?

• Updated query runner tests to account for backfilling initial zero-days.
• 👀

[greptile-apps]

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Total Size: 1.21 MB

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[danielbachhuber]

Looks great!

Just need to fix the "I just launched my experiment" scenario:

It'd be good to have a backend test for that, too.

[posthog-bot]

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• webkit: 0 added, 0 modified, 0 deleted

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