Custom performance metrics support

[pondorasti]

Is your feature request related to a problem? Please describe. One of my use cases for ChecklyHQ is tracking custom metrics through the Performance API. My app leverages performance.mark() to create it's own performance KPI and then check for performance regressions.

Describe the solution you'd like Create an API, probably a lib that you can import, which allows me to define my custom metric value. This will allow the dashboard for each check to display relevant performance data and overwrite default web vital metrics if desired.

const checkly = require("checkly")

const mark = performance.getEntriesByName("demo")[0]

checkly.setPerformanceMetric(mark.duration)

Describe alternatives you've considered So far I've been using performance.getEntries() to programmatically read markers that my app generates, and then send them to a "data warehouse" for processing and data visualization.

Additional context