fix(browser): Stringify span context in linked traces log statement

[github-actions]

Note

The pull request "fix(browser): Stringify span context in linked traces log statement" was created by @s1gr1d but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

I came upon this log Sentry Logger [log]: Adding previous_trace [object Object] link to span [object Object] and this PR fixes this by stringifying the context.

One concern I have with that is that the object could be too large (stringifying takes too long) or circular. But this should be very unlikely in this case. However, if someone else shares this concerns we might change the log to either limit the depth or to only log specific entries of the object (might add bundle size).

[linear]

JS-1243