Allow disabling just tracing on HTTP client requests

[jamescrosswell]

Currently people want to instrument outbound HTTP requests using OTEL, they need to disable Sentry's SMTP message handler to avoid double traces. However this means they lose stuff like the failed request handling... which is a bit of a shame.

Ideally people could disable just the tracing stuff in the message handler... but keep the other stuff that they still want.

Originally posted by @jamescrosswell in getsentry/sentry-docs#15378 (comment)

[linear]

NET-515

[jamescrosswell]

This is implemented in #4899:

• feat: Add support to send OTEL traces via OTLP #4899