JS Loader: Sentry.init should be called - with defaults from docs for Performance/Replay

[HazAT]

As the title says, the goal should be that someone should be able to just paste the <script/> tag and without any manual calls to Sentry.init get the experience using it.
That means, for example, pageload transactions and Replays are automatically captured when Performance and/or Replay is enabled.

It should be possible to still call Sentry.init yourself and with that overwrite our defaults.

[getsantry]

Routing to @getsentry/team-web-sdk-frontend for triage, due by Tuesday, March 28th at 5:00 pm (sfo). ⏲️

[mydea]

So, I think I have the loader stuff working (locally) now. However, IMHO it would be great to have some kind of E2E or integration test in sentry that actually checks the behavior, because this is not really tested as of now and I think very hard to test with the current setup. With the changes, there are basically the following scenarios that have slightly different behavior:

Lazy mode

Lazy mode is only available for errors-only config. Users can manually opt-out of this by adding data-lazy="no" to their script tag.

In lazy mode, nothing happens until an error occurs. Once an error occurs, we download the CDN bundle, and then...:

• If onLoad is configured, trigger it
• If SDK.init() has not been called, call it with defaults

Eager mode

If either performance or replay is enabled, we are always in eager (non-lazy) mode. In this mode, we'll automatically load the CDN bundle, and then...:

• If onLoad is configured, trigger it
• If SDK.init() has not been called, call it with defaults

Defaults

The following default config is defined:

• dsn: Always set
• debug: true is set when debug bundles are enabled
• tracesSampleRate: 0.1 is set when performance is enabled
• replaysSessionSampleRate: 0.1 & replaysOnErrorSampleRate: 1 are set when replay is enabled
• integrations: Will add Sentry.BrowserTracing and/or Sentry.Replay if performance/replay is enabled

This config can be overwritten by the user, and is merged. So e.g. if they do:

Sentry.onLoad(function() {
  Sentry.init({
    tracesSampleRate: 0.5
  });
});

For a full loader build with every option enabled, the actual config will be:

{
  dsn: 'your-dsn',
  debug: true,
  tracesSampleRate: 0.5,
  replaysSessionSampleRate: 0.1,
  replaysOnErrorSampleRate: 1
}

[AbhiPrasad]

We also need to add Sentry.BrowserTracing and Sentry.Replay as well for auto-instrumentation.

[mydea]

We also need to add Sentry.BrowserTracing and Sentry.Replay as well for auto-instrumentation.

Right, this too (actually I did this but forgot to write it down xD) will add it to the list above for completeness!