Change OkHttp events from spans to measurements

[stefanosiano]

Description

We charge performance per span, so we can remove all sub-spans of the http.client span, changing them to measurements. This could also improve readability of the span waterfall. We have a "workaround" that automatically collapses http.client sub-spans, but that's not ideal. Slack discussion: https://sentry.slack.com/archives/GUCRT2B70/p1719330898362759?thread_ts=1719323921.345899&cid=GUCRT2B70

[cleptric]

Span attributes, measurements are something else we don’t want to keep using in SDKs.

[stefanosiano]

We can create a sample and compare the result with the current implementation to check the developer experience