Introduce linear regression experiment on Deployment Frequency chart

Demo

Current gif demo of what's enabled for a few people on production as of 2023-04-14

What does this MR do and why?

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/403630+

This MR introduces linear regression in Vanilla JS for frontend forecasting on charts. The first chart to receive this addition is the CI/CD Deployment Frequency chart.

The linear regression calculation is based off of https://oliverjumpertz.com/simple-linear-regression-theory-math-and-implementation-in-javascript/

Example of the linear regression output:

// Given the following dataset:
const timeSeriesData = [
    { date: '2023-01-12', value: 160 },
    { date: '2023-01-13', value: 52 },
    { date: '2023-01-14', value: 47 },
    { date: '2023-01-15', value: 37 },
    { date: '2023-01-16', value: 106 },
    { date: '2023-01-17', value: 178 },
    { date: '2023-01-18', value: 161 },
    { date: '2023-01-19', value: 173 },
    { date: '2023-01-20', value: 101 },
    { date: '2023-01-21', value: 31 },
];

// And calling 
linearRegression(timeSeriesData, 7);

// The returned response will be
[
    { date: "2023-01-22", value: 113 },
    { date: "2023-01-23", value: 115 },
    { date: "2023-01-24", value: 116 },
    { date: "2023-01-25", value: 118 },
    { date: "2023-01-26", value: 119 },
    { date: "2023-01-27", value: 121 },
    { date: "2023-01-28", value: 122 }
];

The forecasting is displayed by clicking a toggle, at which point the data will then be computed and rendered to the chart to match the design in https://gitlab.com/gitlab-org/gitlab/-/issues/403630/designs/A_Deploymeny-frequency-forecast.png

TODO

• Add linear regression logic
• Add toggle to the chart behind a group based feature flag
• Add alert flow for opt in
• Append the additional data to the chart when toggle is selected
• Change forecast date range based on selected date range
• Ensure tooltips and legend make sense - if applicable
• Potentially refactor the linear regression logic
• Make sure that everything is tested

Open questions

• What should the forecast ranges be foreach deployment frequency date range, i.e. last week, last month, etc
  • Question asked in https://gitlab.com/gitlab-org/gitlab/-/issues/403630/designs/A_Deploymeny-frequency-forecast.png

This question has been clarified:

Last week = 7d + 3 forecast
Last month = 30d + 15 forecast
Last 90 days = 90d + 45 forecast
Last 180 days = 180d + 90 forecast

Screenshots or screen recordings

Width	Forecast
Full	Off
Full	On
Mobile	Off
Mobile	On
Opt in	n/a

How to set up and validate locally

• Feature.enable(:dora_charts_forecast, GROUP_ID)
• Browse to http://{local-instance}/groups/gitlab-org/-/analytics/ci_cd?tab=deployment-frequency
• Click the toggle

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.