Possible adjustments to multipage example

[lenasano]

The multipage example is already excellent! These additions would be to support the full story of the 'instant feature impact detection' use case, in the context of webpage performance measurements. Specifically we hope to introduce:

• the Browser RUM Agent
• automatic metric creation
• viewing metric results
• metric alerting

This is a summary of code changes:

• add two more feature flags to control (1) imgur image size and (2) automation simulated network speed
• demonstrate node.js singleton pattern (rather than duplicate code for creating the Split SDK client)
• adjust the puppeteer automation script so that the INP and CLS measurements are captured

Could we add such changes to the multipage-testing-example?