chore: collect replay network metrics

[vaind]

Adds network info to the collected metrics - it shows the amount of data sent (Net Tx) and received (Net Rx) by the browser. It works by intercepting the network requests with a custom handler that logs the transmission stats. Because Sentry SDK is the only application code sending requests, it equals the amount of data sent & received by the SDK. You can see in the example below that because there are no errors triggered, there's no data sent/received with just the plain Sentry SDK; there's network traffic only when Replay is added.

Currently, only HTTP requests are captured, no file:/// URLs (these are used to load the actual page and scripts from the local filesystem).

vaind#2 (comment)

[Lms24]

EDIT: I accidentally approved this PR and can't dismiss it as it's still a draft. LMK when this is ready for review, then I'll give it another look.

Very interesting! For future readers and because I'm also not a 100% certain I understood correctly what's going on: Could you add a small description of what these additional metrics mean in the context of our SDK/replay? Is it the amount of bytes the SDK sent/received?

[vaind]

@Lms24 it's OK, I just didn't change it from draft back then because I wanted to wait for the CI to pass on my own fork first. Just made the change to a ready PR without any more commits so your review is valid.