[Lens] searchSessionId is reset when opening a Lens config panel in Discover

[mbondyra]

Kibana version:
main

Might not be worth to fix as hopefully we'll do overall fix first (#121543) but let's keep an eye on it as it can cause some problems.

A specific bug:

1. Open Discover in text based mode with a few columns chosen
2. In your code, track searchSessionId somewhere (you can for example add a console.log data.search.session.getSessionId() in src/plugins/unified_histogram/public/container/container.tsx as it is ran on every searchSessionId.
3. Open the Lens config panel and see that the searchSessionId changes when it's open.

Explanation

This is caused by the fact that we initialise searchSessionId for the existing document like this (x-pack/plugins/lens/public/state_management/init_middleware/load_initial.ts):

 const currentSessionId = data.search.session.getSessionId();
store.dispatch(
  initExisting({
    isSaveable: true,
    sharingSavedObjectProps,
    filters: data.query.filterManager.getFilters(),
    query: doc.state.query,
    searchSessionId:
      dashboardFeatureFlag.allowByValueEmbeddables &&
      Boolean(embeddableEditorIncomingState?.originatingApp) &&
      !(initialInput as LensByReferenceInput)?.savedObjectId &&
      currentSessionId
        ? currentSessionId
        : data.search.session.start(),

I would assume it would be safe to just do instead searchSessionId: currentSessionId ? currentSessionId : data.search.session.start(), but there were a lot of bugs in the past with searchSessionId management so it's important to investigate it if we have to fix it. This code should be thoroughly tested and we should also update our test suite because at the moment it's misses few pieces and we cannot fully count on it.

Some cases to pay attention to:

Lens editor initialisation:

1. Opening empty document
2. Opening existing document
3. Opening existing document from the search bar on top of the page
4. When coming from Dashboard -> preserve the searchSessionId from the dashboard
5. When coming from Discover -> preserve the searchSessionId
   Lend editor leaving:
6. Session is kept when coming back to dashboard when saving visualization
7. Session is kept when coming back to dashboard after canceling editing (we have a bug on cancel right now)
8. Session is kept when coming back to discover

Inside Lens workflow:
searchSessionId is changed when:

• changing filters
• changing query
• changing autorefresh interval
• we change something about visualization that doesn’t affect the data but too much time passed (2% of the timerange)

4. Coming from TSVB (not sure what to do here)
5. Autoapply off behavior
6. Sharink link feature

[elasticmachine]

Pinging @elastic/kibana-visualizations @elastic/kibana-visualizations-external (Team:Visualizations)