fix(browser): Mark stack trace from captureMessage with attachStacktrace: true as synthetic

[Lms24]

Background:
Whenever the attachStackTrace SDK option is set to true, the SDK will create and attach a synthetic error alongside events where we couldn't extract an actual stack trace from.

This PR adds the synthetic: true flag to the mechanism of a synthetic exception captured alongside a captureMessage call. Setting this property marks the exception as synthetic which, according to our develop spec, influences issue grouping.

We previously did not include any mechanism in such exceptions, lest did we mark them as synthetic, when calling Sentry.captureMessage(). This PR fixes that.

PR for server/node: #14670

[AbhiPrasad]

Is synthetic: true used in grouping in any way? That's the only negative about adding this right now.

[github-actions]

size-limit report 📦

⚠️ Warning: Base artifact is not the latest one, because the latest workflow run is not done yet. This may lead to incorrect results. Try to re-run all tests to get up to date results.

Path	Size	% Change	Change
@sentry/browser	23.29 KB	+0.02%	+4 B 🔺
@sentry/browser - with treeshaking flags	21.96 KB	+0.03%	+5 B 🔺
@sentry/browser (incl. Tracing)	35.79 KB	+0.02%	+6 B 🔺
@sentry/browser (incl. Tracing, Replay)	73.01 KB	+0.01%	+4 B 🔺
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	63.41 KB	+0.01%	+5 B 🔺
@sentry/browser (incl. Tracing, Replay with Canvas)	77.32 KB	+0.01%	+4 B 🔺
@sentry/browser (incl. Tracing, Replay, Feedback)	89.81 KB	+0.01%	+4 B 🔺
@sentry/browser (incl. Feedback)	40.04 KB	+0.01%	+4 B 🔺
@sentry/browser (incl. sendFeedback)	27.89 KB	+0.02%	+4 B 🔺
@sentry/browser (incl. FeedbackAsync)	32.69 KB	+0.02%	+5 B 🔺
@sentry/react	25.96 KB	+0.04%	+8 B 🔺
@sentry/react (incl. Tracing)	38.6 KB	+0.02%	+6 B 🔺
@sentry/vue	27.49 KB	+0.02%	+5 B 🔺
@sentry/vue (incl. Tracing)	37.63 KB	+0.02%	+5 B 🔺
@sentry/svelte	23.45 KB	+0.02%	+4 B 🔺
CDN Bundle	24.43 KB	+0.02%	+4 B 🔺
CDN Bundle (incl. Tracing)	37.46 KB	+0.01%	+3 B 🔺
CDN Bundle (incl. Tracing, Replay)	72.64 KB	+0.01%	+3 B 🔺
CDN Bundle (incl. Tracing, Replay, Feedback)	78.01 KB	+0.01%	+4 B 🔺
CDN Bundle - uncompressed	71.74 KB	+0.03%	+21 B 🔺
CDN Bundle (incl. Tracing) - uncompressed	111.05 KB	+0.02%	+21 B 🔺
CDN Bundle (incl. Tracing, Replay) - uncompressed	225.12 KB	+0.01%	+21 B 🔺
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	238.33 KB	+0.01%	+21 B 🔺
@sentry/nextjs (client)	38.89 KB	+0.02%	+5 B 🔺
@sentry/sveltekit (client)	36.29 KB	+0.02%	+6 B 🔺
@sentry/node	162.5 KB	-	-
@sentry/node - without tracing	98.69 KB	-	-
@sentry/aws-serverless	126.36 KB	-	-

View base workflow run

[Lms24]

According to develop, synthetic influences grouping, yes. In my eyes, this is still a fix but we can't rule out that it'd create new issue groups for users. I will ask the issues team about this to confirm

[Lms24]

@AbhiPrasad I checked with the issues team and the synthetic flag doesn't influence a lot for grouping, especially not in our case where there is only one generic error type (Error) from our synthetic event. I also sent some test events with and without the synthetic flag. The events were grouped under the same issue. I think it's reasonably safe for us to make this change.

[AbhiPrasad]