Trace propagation broken when there is no continue_trace call

[szokeasaurusrex]

How do you use Sentry?

Sentry Saas (sentry.io)

Version

latest master

Steps to Reproduce

Make file main.py:

import sentry_sdk
import httpx


def main():
    print("Headers in isolation scope (correctly propagated):")
    for header, value in sentry_sdk.get_isolation_scope().iter_headers():
        print(header, value)

    print("")

    print(
        "Headers not propagated on outgoing request outside of transaction (but should be):"
    )
    print(httpx.get("http://httpbin.org/get").text)

    print("")

    with sentry_sdk.start_transaction(name="main", sampled=True) as t:
        print("Headers on the transaction (not propagated, but should be):")
        for header, value in t.iter_headers():
            print(header, value)

        print("")

        print("The transaction headers, not the incoming ones, are propagated:")
        print(httpx.get("http://httpbin.org/get").text)


if __name__ == "__main__":
    sentry_sdk.init(
        traces_sample_rate=1.0,
    )
    main()

Run commands:

1. export SENTRY_TRACE="00000000000000000000000000000000-0000000000000000"
2. export SENTRY_BAGGAGE="sentry-placeholder=asdf"
3. python main.py

Expected Result

Incoming trace is propagated, per spec

Actual Result

Trace is ignored. Root cause is having separate places where we store trace information (#4032)

Headers in isolation scope (correctly propagated):
sentry-trace 00000000000000000000000000000000-8a27591a41b755dd
baggage sentry-placeholder=asdf

Headers not propagated on outgoing request outside of transaction (but should be):
{
  "args": {}, 
  "headers": {
    "Accept": "*/*", 
    "Accept-Encoding": "gzip, deflate", 
    "Host": "httpbin.org", 
    "User-Agent": "python-httpx/0.28.1", 
    "X-Amzn-Trace-Id": "Root=1-67aa3fa3-0049028051b709cd2be65300"
  }, 
  "origin": "redact", 
  "url": "http://httpbin.org/get"
}


Headers on the transaction (not propagated, but should be):
sentry-trace 7c057f50606e45d6a1682a9580c8c978-ad9c29a5285d0d0c-1
baggage sentry-trace_id=7c057f50606e45d6a1682a9580c8c978,sentry-environment=production,sentry-transaction=main,sentry-sample_rate=1.0,sentry-sampled=true

The transaction headers, not the incoming ones, are propagated:
{
  "args": {}, 
  "headers": {
    "Accept": "*/*", 
    "Accept-Encoding": "gzip, deflate", 
    "Baggage": "sentry-trace_id=7c057f50606e45d6a1682a9580c8c978,sentry-environment=production,sentry-transaction=main,sentry-sample_rate=1.0,sentry-sampled=true", 
    "Host": "httpbin.org", 
    "Sentry-Trace": "7c057f50606e45d6a1682a9580c8c978-bf64a1d873991411-1", 
    "User-Agent": "python-httpx/0.28.1", 
    "X-Amzn-Trace-Id": "Root=1-67aa3fa3-6b6554ad237d7d835df81ecd"
  }, 
  "origin": "redact", 
  "url": "http://httpbin.org/get"
}