Separate metric generation from the browser's core logic

[inancgumus]

What

Similar to #4219, separate k6 metric generation from the browser module's core logic. Instead of pushing metrics from multiple goroutines, create a dedicated goroutine to manage metrics.

Why

• Prevent race condition issues, such as #5341 and #4203.
• Reduce flaky test runs, both in k6 runs and CI (i.e., #5367).
• Separation of concerns.

How

• Delegate custom metric management to a new type out of the common package.
• Listen for metric generation events from the core logic and push them to the metric system.
• Research whether these metrics should be pushed on the event loop and in another dedicated goroutine.