Make sure we only ever send a single value in outgoing trace headers.

[antonpirker]

When sending outgoing sentry-trace or baggage headers make sure that they only have one value included.

Something like this never be sent (same goes for baggage header):

sentry-trace: 6a5871d13d8441818ccc593c6c3b07c8-b252c3821bdbaceb-1, 6a5871d13d8441818ccc593c6c3b07c8-88529d2712c386fd-1

See also issue in Sentry where this originated: https://github.com/getsentry/sentry/issues/66701

Related issue for handling incoming headers: https://github.com/getsentry/sentry-python/issues/2802