Align `serialize` span ops

[marandaneto]

Description

serialize should be either serialize.http.client or serialize.file.read depending where it comes from.

ref: getsentry/develop#694

SDK	Old Operation	New Operation	Link	PR
Dart
	serialize	serialize.http.client + serialize.file.read	sentry-dart/dio/lib/src/sentry_transformer.dart Lines 17 to 18 in 7987216 'serialize', description: '${options.method} ${options.uri}',