Allow use of `sessionStorage` in `browserSessionIntegration` to more accurately count sessions for MPAs

[lforst]

Description

Session updates we send from the Browser SDK are currently per pageview, leading to more sessions than is commonly understood for MPAs. We find that Sessions likely should have the same lifetime as a Session Replay session. For this we should introduce an option in the browserSessionIntegration that allows a user to configure a persistence model using sessionStorage to more accurately collect number of sessions.

[lforst]

Strawman proposal: We add a sessioIntegration() with the following options:

interface SessionIntegrationOptions {
  /**
   * The storage type that the SDK uses to store a Session ID that is used for Sessions and Release Health in Sentry.
   * 
   * Possible values:
   * 
   * - `"sessionStorage"` (default) The Session ID is persisted to the browser's `sessionStorage`. Sessions will persist across hard-navigations on the same origin.
   * - `"memory"` The Session ID is stored in memory. Hard-navigations will produce new sessions.
   * 
   * Defaults to `"sessionStorage"`.
   */
  persistence?: 'sessionStorage' | 'memory'
}

The persistence name coincides with a similar/same option and behavior in Posthog. It's just a great name for this option: https://posthog.com/docs/libraries/js#persistence

[bruno-garcia]

Just wanted to mention I think this is a solid plan. Defaulting to sessionStorage with a clear opt-out.

Session Replay relies on sessionStorage and not once we heard this being the reason why someone wouldn't want to use it.

A bigger question would be relying on localStorage. Although that could be an option for users who'd like to have traces connect across tabs, and surviving restarts. But sessionStorage seems like a great improvement to the product without compromising privacy.

[HazAT]

+1 on making it opt-out in other words, makeing sessionStorage default
We are doing this with the intention to provide a better service, it is communicated in a major and we are not storing any sensitive data in there so a very clear case for me

[lforst]

This has made another round among stakeholders, we will have to make the usage of sessionStorage opt-in. Which will make this a non-breaking change and not part of v9.